I bought a chip at the local Circuit City for mine since I had to have it 'right away'. It's never given me any problems as of yet...well over a year or so. Brand is Centron. Cost around $110, but I got $30 back from a mail-in rebate. I've always used aftermarket ram in my Macs, never an issue. Just make sure the specs for what you buy match Apple's specs.
You don't need more than PC100 in an iBook G3, so just pick whichever is the cheaper...probably the PC133. Just make sure it's backwards compatible (most memory made todays is).
Just a fair warning though, depending on how you'll be using this, you may not get rid of the beachball problem totally. The G3 models are great computers, but once you start mid to heavy weight work on them they're just not capable enough. The extra RAM will help for light use (internet, mail, word, etc.) but don't expect it to be a great performer with Photoshop or InDesign. I pageout so much it isn't funny right now...could really use another GB of RAM (640mb currently, 651778 pageouts in 6days, 10 hours, and 52 minutes).