Hello and Welcome to Apple Discussions,
Well, the devoted Clamshell forum on Apple Discussions is
http://discussions.apple.com/forum.jspa?forumID=927. It's hard to see on the iBook forum page. It's up at the top, above the iBook G4 forums.
For your upgrade question, I would suggest iFixit's Clamshell upgrade page.
http://www.ifixit.com/Categories/Clamshell/49
Another good place for RAM or Hard Drives would be PowerBookTech. They do not have Specific Clamshell upgrades, but compatiable parts mentioned in my last sentence can be found in the iBook G3 12" White area of PB Tech.
Macrecycling.com has a variety of parts like PB Tech. Like PB Tech, there is not specific Clamshell area, but compatiable parts are found in the White iBook G3 12" section as well.
WeLoveMacs.com has a nice Clamshell section. They are extremely reliable, but from buying a lot of parts from them, their prices can be too high for many parts. (example- iFixit may have the part you need for $49, and WeLoveMacs wants $149. I'd shop around before buying from there. Why pay more when your getting the same part?)
I wouldn't buy parts from eBay, even though it may sound like a good deal, it is better to buy a part that is warranted from a trusted place and is more reliable.
Other resources are buying Clamshells and using parts from them. I have seen 466 Graphite iBooks go for $100 and iBook parts in Boston, MA at the Massachussetts Institute of Technology Flea Market. If you live near there, you can go on www.MITflea.com for more information.
Hope this helps, and good luck.
-Brian