Got it from a dude who bought it new for a project then parted out the project before he ever installed it. He took the loss for me. But they can normally be had pretty cheap at www.walbro255.comhttp:// or eBay.
I think that happens mainly on eBay. I think Walbro255.com is reliable though. I've heard of a lot of people getting theirs there and they claim that they don't sell knockoffs.
There's math formulae to figure fuel requirements for given hp numbers. I don't know any of them. Generally I think people just shoot high for their application assuming that the regulator will return any excess back to the tank. The "standard" is the GSL392 for most of the 4x4 guys with V8's.