I suppose it depends on where you are and it might include a bit of luck. Here in DFW I found a source after calling some auto shops that deal in performance parts. One gave me the name and number of a guy who supplies nitro and methanol to the "gear heads", race tracks, etc. I usually meet him somewhere, like a convenience store, and he fills up an empty gallon can for $25. One gallon lasts me a long time since I rarely use over 5% nitro when I mix my own.
I get the methanol, $3.75 a gallon, from a local auto parts shop.