First, you need to know which transmission you have:
A.T., 3.8L, 3.05 ratio F83, ident LBB
A.T., 3.8L, 3.29 ratio FR9, ident LCB
A.T., 3.8L, 3.29 ratio FR9, ident LNB
A.T., 3.8L, 3.05 ratio F83, ident LPB
Those are all listed for a 2000 Impala by the car-part Interchange.
Now, if you want to try for a decent used one, give www.donsauto.com
a try. They are actually right down the road from my work, and offer a 12 month/12k mile warrenty on their major components (Engine/Tranny/driveline/etc) and I know many people around here use them. All their vehicles are under 95k on them. And I know they have an LS in there (even though not shown on the site) cause I can see the front clip on their rack in front (was looking for a white fender and it was silver.)
Just a thought. Price would be lesser than the ebay one you mentioned I believe.