I'm in the middle of doing this myself, but due to a family member being in the hospital right now, having to put it on hold.
I spoke with several people at various places recommended by my mechanic. You CAN have it done commercially. Expect to pay from 100 to 400 dollars and in some cases add shipping.
OR
You can contact the POR-15 folks, and get one of their Fuel Tank Saver Kits. This includes the Cleaner to get rid of the Varnish and other stuff, the Metal Prep to etch the metal and the Sealer.
Don't expect this to be a one day job though. Just from reading the instructions you are looking at least a full day just with the cleaner, then another half day with the etcher and then, depending on your weather / temperature another 3-4 days for the sealer to work.
Cost? Try $50 + shipping.
I'm sure that you can find a name of a POR dealer near you, if not my contact for it is:
Custom Product Sales
3115 Crocker Road
Eugene, OR 97404
(541) 345-8092
Chuck is the guy's name, but don't know his last name.