I like what Joe Brancato (JB) airtanksforsale.com/ExpertHPA has to offer. I just received my new Guppy 18cu.ft. tank from him recently and just got it filled today at a local dive shop. The second gauge helps if you want to know how much air is left inside your tank instead of using a deadhead. A little pricey but good valves made in the US. His tanks come with DIN300 threads which all dive shops should be able to hook up to and fill.
DIN300 threads;
DIN300 to male QD foster adapter;
2nd gauge to tell how much air is left inside the tank;
There's a few things you should know about getting an SCBA tank filled at a dive shop. You can ask them to fill it to 4500 psi/310 bar. If they do a "hot fill" (filling it fast), when you get your tank back from them if they just finished filling it, it will show 4500 psi. A few minutes later because of the heat created from filling too fast, your pressure will start dropping.
By the time your tank cools off, you will lose about 200-500 psi depending on how fast they filled it. You can ask them to do a slow fill but that means they have to stand there and take there time and monitor it. If it's a busy shop, they might not have the luxury to do it. You can also ask them if you could come back later to top it off to 4500 psi after it cools off maybe in the next day but they might charge you again.
Having them fill a brand new tank with no air in it, I'm pretty sure they're going to do a hot fill. When you need a refill at lets say 3000 to 3600 psi, you'll won't see a big loss in air pressure if they fill slowly. If you got a young kid manning the fills, then forget it. I guess that's joy of investing in an HPA compressor.