What's the most recent version of OS X I can run on a 1999 iBook and where can I buy it?
The maximum OS version is 9.2.2 for Classic Mac OS or OS X 10.3.9 (Panther). I'm not sure which one to recommend. OS 9 might be faster, but OS X is certainly more powerful and will be far better supported by software (though Panther is so old that you'll have to use old versions of many apps). 

You'll probably have to go digging on Craigslist or eBay for install discs that old.