How can I buy the lower case screws?

They are likely metric. Easiest thing would be to search laptop base screws and see if you find a match.

Or you could remove a good one and go to a hardware store, like ACE, and match it up with their selection

If damaged means they won't screw in or stay in, then the problem is likely broken plastic that they screw into. New screws won't help for that. New case pieces would be required.