In order to grow giant beans, you need to pick out a variety that grows to an exceptionally large size. The two types of giant beans that I have had the most success with are Yard Long Asparagus Beans and Red Noodle Beans.
I personally have never purchased from them, but it seems Kitazawa Seed
has a good selection of giant beans.
The seeds I used were from the clearance of a Ripley's Believe it or Not museum gift shop (5 cents per packet) and also from Territorial Seed Company
. Chances are you won't find these beans in the seed department of your local Home Depot, so the internet is probably your best bet for buying seeds.