macmac,
Your Hardware Maintenance Manual is:
ftp://ftp.software.ibm.com/pc/pccbbs/mo ... 2p2252.pdf
On about page 218 you'll see a schematic of the notebook and can find the actual part numbers below that. There's a lot of base cover models available, so be sure to locate the correct one for your model. You may also want a screw kit, 13R2916 and possibly the base misc parts, 13N5104. I personally recommend searching Google for these part numbers and ordering from whoever is cheapest. Getting them straight from IBM probably would be more expensive.
I've replaced some plastic on my T30 before because of stupidity (I tried to use a chemical to clean off the sticker residue from the Pentium/Windows stickers and that chemical stained the plastic) and it can be intimidating, but if you are careful and follow the manual everything will go fine.
Also, if you need any help deciding exactly which parts you need, post more pictures and we'd be glad to look.