where can I find a rear subwoofer for a 1991 Lexus LS 400

try a salvage yard, a radio supply house or the dealership.

The car has nothing to do with the sub besides what will fit in the trunk. If you want a lot of trunk space then l would suggest going with maybe a 10'' sub in a sealed box. Which ever sub you choose of that size will mostly depend on your budget. Either look at your local audio store or ebay dealers will give you a better deal.

checkout http://reichardtim.tripod.com/