Not quite sure what you mean, a quick search on google shows quite a few 9v 2amp owner supplies.
or go 12v less amps?
I've also had 4 axos running off a power hub, that's was powered via 12v 2 amp
Also I dont think you'll need 500ma per board, as it's unlikely all 4 will draw 500ma, and if they do isn't that at 5v?
Axoloti seems pretty 'open' to different power options
Btw if your looking for a supply that has multiple outlets, perhaps look instead for a power daisy chain
( though double check their current rating)