a little while back I would have said in one of the 5 gallon buckets out in my FIL's shed.
I don't know where he got them all, he surely didn't have them when we moved him up here.
I'd go talk to your local ACE store, mine carries a small assortment of screws for guns not too many.
but they do have a contact somewhere and could probably hook you up for like 10-15-20 cents a screw if your willing to wait a couple of days.