Make thorough notes in the script of every prop that is required and compile a list.
Leave space after each item on the list for the names of members of your cast and crew.
Bring the list to a production meeting to find out who among the crew might be able to find each object.
Bring the list to a table read to find out who among the cast might be able to find remaining objects.
Visit flea markets and second-hand stores. Drive through residential neighborhoods on weekends looking for signs with directions to garage sales.
Search the internet or your local Yellow Pages for novelty stores, costume shops and prop stores to find the remaining props that haven't been found yet.