The Cramps' spiritual leader and guitarist Poison Ivy is also well known as a collector and connosieur of rare rockabilly and garage-rock singles. These two CDs collect the 50's and 60's obscurities covered, touted, or quoted by The Cramps.
Songs The Cramps Taught Us: Vol. 1 (104 mb, 192 kbps) and Vol. 2 (99 mb, mostly 192 kbps) on megaupload.
There is also a third volume that I don't have, I will appreciate any leads on that. Marcel in the comments pointed me to the Wild Wild Party blog that has the third volume - along with a ton of other great stuff! The track list is in the comments.
P.S. A lot of the same material is covered on the Born Bad series.