I've found and purchased a used differential on this site:
Car-Part.com--Used Auto Parts Market
The secret to finding one is to look for "axle carrier" instead of differential.
Call the yards that advertise they have one, confirm via VIN that the donor vehicle is correct. Expect to pay a little extra for shipping.
I had to replace the ASD differential on my '95 sedan, I think I found one for $200 or $300, had the yard verify it was still full of oil and turned easily before shipping. It's been in my car for a couple years now no problems.