I have had a shoebox for 15 years.
Works great but sadly they closed shop and no parts/rebuild kits available.
I use it to fill my four fire fitter tanks.
Best purchase I made,but there will be a time it needs replaced.
Our 4H club has a booster pump, gifted to us from the NRA Foundation. It requires a feeder compressor that can run 5? or 6? cfm at 80psi, so a lot of conventional home use compressors can feed it. The trick we learned was to refill our scuba tanks when they get down to about 75% capacity. We don't have guns that feed direct from compressor.
Just so you know several other brands are built by vevor. Mophon comes to mind. If you want a full rundown on mine shoot a pm and we’ll trade phone numbers.
I have a Yong Heng (similar to vevor) and while it's not the most intuitive or quiet tool in the world it works great for direct-gun use. I'm not sure I'd trust it to fill a large scba but a smaller buddy tank I'd not hesitate at all.
If you're looking to fill full size tanks - check out govdeals or publicsurplus and try to get a decomissioned fire dept compressor. Watch out for ones with particularly high hour count because sometimes the compressor rebuild kits are discontinued, but sometimes they just get sold because they're no longer recertifiable (for human breathing use).