Q: Are there any portable printers that will print directly from an iPad?

No. AirPrint compatible only >  About AirPrint - Apple Support

Take a look at this article http://www.macworld.co.uk/how-to/ipad/how-print-from-ipad-iphone-wirelessly-3459 464/

Bluetooth is supported for stereo headsets/speakers, handsfree telephone devices/headsets, some keyboards,

some peer-to-peer apps from the app store and internet tethering where provided by the carrier.

Other than this it will not connect to a phone/computer/device.  (thanks to ckuan for the wording)

As provided in previous answers, you will need an AirPrint capable printer.

You can print from non-AirPrint printers if you use the manufacturer's app. I have a non-AirPrint Epson and do so from my iPad. However this will not solve your issue as it still requires Wi-Fi. You don't have to have an active Internet connection but you do need a local network.

You do not need the internet for wireless printing.  You need:

-- wifi router

-- airprinter.

You can print to certain bluetooth printers with a helper app

Helper app -- /print n share

Print directly to the widest range of network or WiFi printers available in any app. Alternatively print via your computer to ALL printers including USB & Bluetooth printers. View read and display large PDF, Office & iWork files. Easily transfer & store files from the Cloud or your computer.

https://itunes.apple.com/us/app/print-n-share/id301656026?mt=8

Mobile printing.

probably the best solution -- HP Mobile 100 or HP Mobile 150 printers using HP 1200w NFC

https://www.youtube.com/watch?v=9zVfCpT4X84

Brother PocketJet® 673

-- Prints on a variety of 8.5" wide thermal media.

-- Print PDF files from your iPhone and iPad with the PJ-673 Print app

http://www.brother-usa.com/Mobile/ModelDetail.aspx?ProductID=pj673-k&PG=26#.UeX0 Z18pDDJ

http://www.brother-usa.com/mobile/lp/pocketjet7/

"Mobi Print gives its users the capability to use iOS devices to accomplis iOS mobile printing, such as iPads, iPhones, and other Apple products, to print with any of Zebra’s mobile, CPL, & ZPL printers. "

http://www.arrowheadphx.com/mobi-print/

"Mobi Print gives its users the capability to print (bluetooth or wifi) PDF's, Images or Cloud based label formats to Zebra mobile or desktop printers on iOS devices."

https://itunes.apple.com/us/app/mobi-print/id701088704?ls=1&mt=8

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List of supported AirPrinters.

    https://support.apple.com/en-us/HT201311

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R