I ran across http://www.sanidumps.com/ today. They have quite a long list of dump stations.

I also have a book titled RVer's Guide to Dump Stations, which has fewer stations listed, but is very useful since it lists them based on interstate and exit number.

On other book that I use all the time (not for dump stations) is called The Truckers NAT Service Directory. I lists all the truck stops based on which interstate and exit number you might be near. I use this one all the time. I picked it up at a TravelCenter Truck Stop a while back.