I want to purchase a current printer for an old Mac.

You won't find a new Mac-compatible printer that has a mini-DIN 8 serial printer port (which is what the 6200 needs) and pre-OS X drivers. A Performa 6200 is now 20 years-old, and that's ancient in terms of its technology. Modern developers aren't interested in writing drivers for such dated hardware. Unfortunately, a Performa 6200 is one of the least desirable PowerPC Macs ever made - and your Mom was unlucky enough to get one that required the L2 Cache/ROM DIMM card replacement. Mine didn't and after years in storage, sits in the basement awaiting its e-cycle fate. The PDS slot is useless for current connectivity options. You'll need to find a used printer and my only recommendations would be the Epson Stylus Color 740 and Stylus Color 900, both of which were released in 1999. I have both, and while the 900 was a more expensive and better printer, it's more difficult to find replacement ink tanks for it than for the 740. Both printers offer the best of all connectivity options: Parallel and USB for PCs and USB and a mini-DIN 8 serial port for Apple computers. Other than ebay, you might want to check any thrift stores in your area that accept donated computers and related peripherals. Beyond their text printing qualities, both of those printers do very good job printing color photos. I'd avoid looking for a used Apple StyleWriter 1200 (non-color) or Color StyleWriter 1500, 2400, or 2500, all of which are Canon-made printers. The Epson models that I've indicated are much better printers than any of them.

Does she have the computer on a home network? If so a PostScript-capable laser printer is the perfect choice for these machines.

Since the StyleWriter 1200 is a black and white printer, an older monochrome Apple laser printer could be another possibility. There are several models to choose from (intended for LocalTalk or a direct serial connection). The print quality is usually still very high, and the cost per page is low.

As a temporary workaround, one can install a free printer driver called PrintToPDF (http://www.jwwalker.com/pages/pdf.html). That would allow PDF files to be created. These files could then be moved (on floppies) to another computer connected to a modern printer.

Addendum

ryan42 mentioned PostScript. Many of the Apple models (but not all) are PostScript printers. These can be connected to a LocalTalk network, or in some cases to Ethernet.

There are also certain HP laser printer models for LocalTalk.

A LocalTalk network can be very simple, with just one printer and one computer.

Some Apple laser printers (without networking capabilities) use another printer language.

If you wanted an older Postscript-capable laser printer, the HP LaserJet 6MP is a good choice and can connect to the (mini-DIN 8) serial port on a 6200. You use the Apple Postscript driver for it. Replacement toner cartridges for it are still widely available in office supply stores without special order. The LaserJet 6MP is HP's LaserJet 6P (designed for use with PCs), converted to a Postscript printer with a PostScript SIMM card installed in one of the (3) memory slots. I have a 6MP and also converted a 6P to a 6MP for use with my older Macs. They're very capable laser printers, although slow by today's standards. If you wanted an Apple LaserWriter printer (and I'd be reluctant to buy a used/high mileage unit in 2015, given their age), the Personal LaserWriter LS, Personal LaserWriter SC, Personal LaserWriter 300, and LaserWriter Select 300 use the QuickDraw printer language protocol and all (but the "SC") can be connected to the serial port. This distinguishes these models from the rest of the Apple's laser printers that are Postcript printers. In addition to the cost of a used laser printer, the shipping expense will be high, given its weight. The replacement toner cartridge won't be inexpensive either, as the price usually starts at $80 (USD) and goes up. If your mother were running a home office with moderate-heavy printing demands each day, a laser printer would eventually pay for itself. If her printing needs are more typical of the home user, I'd still recommend a used Epson StylusColor 740 inkjet printer. You can find replacement ink tanks at Target, Office Depot, and many other retailers. Keep in mind that you'll likely spend at least $60, to replace both the black and color tanks.

I second the LaserJet 6MP suggestion. I still am using my 6MP as my black-and-white printer at home. The issue you'll likely run into with the Epson inkjet printers is that they have an integrated print head that's likely long been clogged from non-use and storage, but you could get lucky. The HP inkjet printers of the era had disposable printheads (like the Deskjet 870) so those may be more reliable so many years later.

The other issue for inkjet printers is finding in-date ink cartridges. My 870Cse uses a 45 black cartridge which is still available, but it uses a 41 color cartridge that is long discontinued.

The LaserJets still have toner available for them and have very good print quality provided you don't need to print in color.

A specific suggestion:

http://www.ebay.com/itm/271793444326

Other Serial Printers by Epson:

Stylus Color 740 (and the 740i, colored like an iMac)

Stylus Color 850

Stylus Color 850

Stylus Color 1520

Stylus Color 3000

Stylus Photo 700

Stylus Photo Ex

Stylus Pro 5000

Stylus Color 900

Be careful trying these down because some of the "Stylus Color" models have the same number as "Stylus Photo" models that are NOT serial-capable.

Serial Printers by HP:

DeskJet 870Cse (the only one I know of).

In the interest of technical clarity, you want a printer that connects to the serial port, which mates with an 8-pin mini-DIN plug. That's not the same as the 4-pin ADB (Apple Desktop Bus) connection, which is similar to an S-Video plug. As for the Apple Color StyleWriter 2400 & 2500 models (manufactured by and based on the Canon BubbleJet 4000 series), I ran into problems on two occasions with almost-new replacement cartridges. Two Canon replacements (and not from the same production run) developed problems with the circuit board attached to the side of cartridge. With about 80-90% of the ink still in the tank (by weight), I'd get an unexpected error message when attempting to print. "The ink cartridge is either missing or not installed correctly" was indicated. This was a common problem with those printer cartridges, so when I moved on to the Epson printers, I never considered buying another Canon. I give your mother credit for sticking with that Performa 6200, which has some inherent design flaws that can't be corrected. If you had replaced it with a used beige G3 running OS 8.6 about 10 years ago, she'd realize how s l o w the 6200 was by comparison. You would have had the option to install a USB PCI card, which would have opened the door for many printer options, not to mention the use of other USB peripherals, like flash memory sticks. It wouldn't help with a 2015-vintage USB printer lacking pre-OS X drivers, but you could have upgraded from the StyleWriter 1200 in the early to mid-2000s and found a printer with legacy drivers.

Where are you geographically? I know where some Epson Stylus Color 740s are. I could send you one for the cost of shipping if you want but it would be after next Saturday.

I could copy the drivers to a floppy disk or CD and include them with the printer. That would be a non-issue.