What Do You Guys Use Under The Carpet?

[69sixpackbee]

I have to repair the surface rust on the floorboard and then re-install the carpet. The underlayment, however, is trashed. I really don't want to put the original type of tar paper/foam underlay back down and I don't want to lay down the ungodly price for the Dynamat crap either. What is a good heat resistive and sound deadening underlay to put down? What have you guys used? I am going to hunt around the McMaster-Carr catalog and see what i can find too.
Thanx!