Just did this, I wouldn't bother with used, just call aa, you'll need an install kit also. It was less than $200 with the different adapter for the 6l80e. I ordered it all through High Angle Driveline.
If I were to do it again, I'd ask them about getting rid of the 6 spline internal junk. Seems silly to have a beefy 32 spline input that necks down to a 1950s 6 spline junk.