There is a ceiling on hard disk drive capacity in older Macs, so be certain to read up on what that is before getting an ATA/IDE (parallel ATA) replacement hard disk drive. The bus speed is slower on those, so hopefully an ATA100 can work, though the computer spec is less. Data speed is also way slow.
A source of a few models, some above limit in iMac G3:
http://eshop.macsales.com/item/Seagate/ST3120213A/
http://eshop.macsales.com/
Probably would not be worth the expense to put a SSD Legacy
OWC mercury in an iMac G3, but they have them for G4 Mac.
Since USB 1.1 is slow an external drive would be limited for Mac OS, and it would not support booting a clone or system copy should you be able to run an early version of OS X in addition to Mac OS 9.2.2 & before. An external with custom oxford-chipset will work to boot OS, via FW.
For external hard disk drives, see if your computer has FireWire 400 ports and look for an enclosure that supports booting Mac systems, and has its own power supply since the Mac ports can't be relied on to power an external hard disk drive reliably, least of all an older model. And hard drive data rate is very slow in these older Macs with drives inside the computer, and slower if accessed externally via USB1.1. The FW400, if available, is significantly faster.
If you get an enclosed hard disk drive with both FW400/800 and USB2/USB3.0 that would work OK with an older Mac. See the OWC pages for these; or call for advice. And note the speed the computer can access data to and from a hard disk drive is the bottleneck, so too fast of a hard disk drive and the drive will not be compatible.
Good luck & happy computing! 🙂