If it were me, I would make one. If you want to buy one, check out a company named "Calex". They sell really high quality, low ripple, low voltage power supplies that come in very nice small packages. Don't know if they make one exactly suited to your needs in terms of current, but for sure they make 6V supplies. Cost around $100-$200. You might also check eBay to see whether anyone sells 6VDC regulated power supplies for tube filaments. Possibly there is a Chinese source for a cheap but decent supply. I'd go Calex.
Custom Power Supply for 6v DC Motor
I'm trying to find a more substantial power supply to replace the wall wart that came with my Amazon Model Two. I need a 6v ac/dc converter. Anyone know a source for something like this? Or someone that could make me one? I can get a Black Lightening from Red Wine Audio but that's $1000 plus. I'd be willing to spend half that.