[ksarul]

The keyboard itself shows up on eBay quite often for between $5-$10. You might also want to ask on the TI subforum over at AtariAge--several folks over there have mentioned having dead keyboards that you may be able to get to pull the replacement part from. One note: there are about half a dozen variants of the TI keyboard, so knowing which of the keytop types yours uses would make it easier to figure out if anyone has a functional replacement part for it. Some had a square hole in the keyboard with some leaf springs in them, others had a plus shaped top on the lower key base. All of the variants end up as one of those two general types--although some are black and some are cream colored. Note that you'll also need the space bar spring it it is missing.

Here's one eBay seller that often has loose keys for sale as well--you might want to message him to ask: buddybear99

I hope this helps. AtariAge has a really active programmer community on it. There are also several active TI groups on Yahoo! There are also plenty of new items available for the system (some of which need no expansion beyond the console you have).