I just went to both ACE and Lowe's hardware stores.
They had screws that looked like they'd be the right size, but even though I took the needleplate with me they admitted they were guessing.
I bought 3 different size screws but none of them work. One is soooo close but just a teeny bit too skinny.
But I did find someone on ebay who'll sell me 2 screws for $12 and postage!!
I'd hate to pay that much though.
I guess I'll just keep sewing without the needleplate screwed down. It doesn't seem to affect anything.