Greenleaf Auto Parts, here they are:
http://www.greenleafauto.com/
When Jerry W. discovered that my Reinhart "Bulletproof" Trans was bogus, and just a '94 stock trans, and Jerry needed a late model core quick to build up for me, I located an '02 4r70w in the local Michigan Greenleaf location for $600. Jerry told me that when he opened it up he found no wear at all on the clutches. It appeared that it had been removed from a car that had somehow been flooded.
Several others I know have found low mileage r4's (and engines) from Greenleaf... $600 or so shipped seems to be the going rate. You'll want one that's at least 1999, but 2002 and up is better.
There are other sources, but I know you can get something at Greenleaf.. they have a warranty/guarantee policy with their parts as well..
-mike
1996 Thunderbird LX - Presently Morphing in Atlanta
Renegage Heads / CompCamns / OEM Cork NPI intake
245rwhp
13.71
&
1996 Thunderbird LX - Pure stock (well almost)
&
2000 Corvette Convertible LS1