A discussion about where we get our airsoft guns and where you might find such prop weapons.

As a summary:

Spring guns - Cheap, no blowback, plastic.
Gas blowback - More expensive, requires 'green gas' or 'propane,' metal
AEGs - Most expensive, heavy, looks great.

Check online for airsoft websites or local airsoft retailers. Hit up your local sporting goods stores for potential props. And ALWAYS INFORM AUTHORITIES when you're filming with these things!