So, basically just call local FDs and ask if they would fill a scuba-tank with 3000PSI of air for a reasonable price? (My animal-brain is telling me the FD would consider that an unusual request, but I'm probably over-thinking it.) If they act confused, I guess just tell them "my scuba-buddies do it all the time, I'll pay $10/tank same as my local dive shop."
I'm about 1-hr from the nearest scuba-shop, and don't have the space for a fill-station. My backup-plan was buying a bunch more SCUBA-tanks, and just filling 6 to 8 tanks at a time. And timing my dives so I can fill, dive near the shop, and fill-again on the same day, before driving home.