Check the online Mac shops like MacMall or MacSales. Low End Mac (http://www.lowendmac.com) has a list of Mac resellers also.
Any of these stores will have PCI video cards that will work on your G3 Macintosh. I would recommend the ATI Radeon 9200 Mac Edition. You could always get the PC version, but as has been covered in other threads about flashing PC video cards to work on Macs, it's better to just get the Mac-compatible one.
You could also check eBay and see if they have said cards for much cheaper.
Also, if you want to take the risk to flash a PC NVIDIA card for Mac use, check here, but do so at your own risk...
For anything else, check here:
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1