I went to The Source today, and I can confirm that their Nexxtech gas is indeed tetrafluoroethane. They sell 2 sizes, one for $13 and the other for $16.
I bought a can and it works great in my WA Beretta. Well, it worked great for 2 fills then the filler tube on the can of gas broke off, and is impossible to replace. Looks like I'll be dropping by The Source again tomorrow to snatch up a few more cans.
Also, I ordered a few cans of the MG Checmicals gas from here:
http://www.ccrwebstore.com/shop/prod...products_id=95
5 cans for $54.15 US, including shipping.