Check out insulation suppliers. Also foam manufacturers, such as "Handi-Foam" or EnerFoam".
These guns are neither inexpensive or durable (they are in effect disposable units no matter what anyone tells you). The life expectance is somewhat based on how frequently you use it. If you leave it for more than a few weeks unused, you may end up throwing it away. Even with frequent use, I figure a year out of one is just about the best you can hope for.