Eclipse SE8262 - might not find on Ebay, probably about 75 if its on there.
Alpine SPS-1629S - probably sells for about 60 on Ebay.
Kenwood KFC-XR601 - more expensive, however probably the best shallow depth mount speaker, probably sells for about 100 on Ebay.
All of these will fit the rear and give you the sound that you are looking for.
BTW, the fronts and the rears have 1.75 depth, usually not a huge deal with 4x6's, but still important. The matching 4x6's for those 6 1/2's are as follows
Eclipse: SE8462AAlpine: SPS-4629 OR SPS-4629P, the latter of which I recommendKenwood: KFC-X468C