This may sound stupid but ho do you approach your local FD to ask if they do fills? Just call up or stop by a firehouse and start chatting?
basically, but they aren't going to offer, so don't go in swinging. Start talking to them, get to know the guys, etc etc. If they have a rescue dive team look at volunteering, make donations etc. They are highly unlikely to have a scuba fill adapter, so you'll have to have your own, but it's kind of a delicate subject to approach. knowing a guy helps, so try to figure out how to know a guy first
@sealark that was probably the gas motor as opposed to the compressor assuming the balancing weight was the right one and the fans weren't damaged. all the ones I've used have been quite stable, but they are powered by lawnmower motors so the gas motors aren't going to be quiet
We've taken our kids to the fire station a couple of times for tours. If you have kids (nieces, nephews, etc). Call and ask if you can bring your kids by to see the fire trucks. They're usually happy to do that. Then just kick up a conversation about air tanks. If the muni fire department doesn't work, try a local/rural volunteer department.