www.corral.net www.stangnet.com these are the two big ones, w/ a few others scattered around.
To be completely honest, you will need a bunch of stuff to swap in a 5.0L V8 properly. Thankfully pretty much everything is readily available either new, or out of a junkyard, and is reasonably priced.
Ideally you would want to find a wrecked 5.0L fox body that you could cannibalize for parts, including engine and driveline parts, as well as any updated parts you may want (nearly anyhting from 79-93 mustangs will bolt right on to your car, and even some 94 to present mustang stuff)
It all comes down to how much you wanna spend and how much work you wanna do.