Go to a hardware store.
You'll have better luck at an auto parts store, although some of them may be specialized enough that you 'll need to get them from the dealer. By the way, they are probably bolts not screws. They'll take you a bit more seriously at the parts store if you use the right terminology. I hope you have at least one original one to show them or you'll find they won't be able to help you. Also an auto wrecker may be able to help you out.
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