Blank guns are available through many collector shops, both online and through mail order. They are also known as "Replica Guns". They can also be purchased through some hobby shops who specialize in airsoft.
Prop guns for film can be purchased at locations such as, costume and theater shops that sell professional quality props or web-sites that specialize in props, especially weapons like guns and swords.
Many of the movie studios sell props from films after the films have been finished. Check with the movie studios and see who might be selling what, or find out where they get the stuff made.
Surprisingly, you can often find prop guns for sale at auction houses and the like. Studios occasinally sell off all their old props to free up space in their warehouse.
