Are you talking about bulk, or just getting bottles filled?
If it's for fills check the local Checker Autos. I've found that typically some of the smaller towns will have one store that will refill. No guarantees though. If you're physically in that area, you can try asking a kid in a ricer too. Even if they don't run it in their car, they'll usually be able to tell you where kids go to get it.
There's a 1-800 number, but it doesn't list all the places that carry it. In fact when I called it for Phoenix it didn't list any of the places I usually refill at. 1-800-997-3345
If you're looking for bulk try welding supply places, or bulk gas sales. They may not carry it, but might be able to order it for you.