Do a Google search for "phosphoric acid [your city]" and you should come up with a short list of industrial suppliers.
If you're in a small town you can shop for a supplier online.
Amazon.com is another good place to start. They are brokers for just about anything.
Stores like Home Depot carry phosphoric acid in gallons or quarts but it's expensive and I wouldn't trust the strength.
Sources like Naval Jelly have just found a way to package phosphoric acid in a gel form so it will stay on a vertical surface. Eastwood sells a similar product in gallon jugs.
Good luck,
