I have the same question? Where can you buy new screws. Apple put in these rediculous tri-screws.... The short one went in crooked and jammed. I couldn't get the long one back in at all, not sure why, but it's not catching a single thread.
My 2nd question is, what the alternative if you can't get the screws back in (e.g. some kind of tape). With only one screw only half way in and jammed, the battery doesn't seem to be moving around, so I think I'm ok, at least for now.