I just get the cheapy type saddle pads and have yet to have a problem from any of them even riding horses hard every day, I never notice any of them with a sore back. Not to mention they are easy to clean and cheap to replace.
This is the brand that I have been using for a while and am pretty darn happy with. Apache Southwest Saddle Pad: Saddles Tack Horse Supplies - ChickSaddlery.com
Only bad thing I have to say about them is that I wish they were 32x32 instead of 30x32
ETA: I just found these that are bigger but still nearly identical (not nearly so colorful though). http://www.chicksaddlery.com/Merchan...gory_Code=1030