Just out of curiosity--what happened to the one you need to replace? Differentials are tough, and it takes some effort to wreck one. That said, the PO of the car I'm restoring seemed able to mount that effort. Running the diff without oil was a good start.
I just rebuilt my TR4A differential (solid axle car), and the experience showed me clearly that there is no such thing as an inexpensive, good, rebuilt differential. Anyone who can do it in a day is a genius, or at least very experienced. So, the guy who rebuilds it has to be compensated for that amount of time and, of course, parts.
Now, as to where to get it-- I dunno. But be sure that whoever sells you a rebuilt one has replaced the bearings and seals, and knows how to set it up. I think I'd question the seller very closely about that. A used one might be OK, but I think it would be a good idea to open it and check at least the backlash, pinion engagement with the ring gear (with dye on the teeth--a standard operation), and make sure the bearings are OK.