Photo below. I bought this Area 51 sign brand new (but manufactured to look weathered and old) for my X Files pinny or for my Revenge From Mars as a topper.

Was like $25 and is the size of half a carton of beer. So its sized extremely well for a topper (not too big, not too small). I just got lucky.
I think it was at a gas station somewhere on a shelf, so I just grabbed it.