Try some triatholon retailers or forums. I've got a post moderne tri post that offers 44mm of setback. They've been offered over the years by Nitto, Easton and FSA among probably others. Keep looking and you'll find one. Also look for saddles that have a longer rail.

If this is to fix the relation of your leg to the crank, then you need a set back seatpost. If its to increase the size of your cockpit, then you'd do better with a longer stem. If you need more than 1.5" anywhere, you need a bigger bike.