There are a bunch, some are more expensive, some are less. You can tell what I use - it works and it's pretty good value. Kolossus is 100mil. I've heard other people say there's some stuff sold in small tiles rather than large sheets. I've also seen a site where they do something different, can't remember what it was - does anyone else?
In my eyes - if it sticks, lasts, resists vibration, and withstands high temperatures, should be good to go. I'd also look for value. Stuff like dynamat isn't cheap and most other options that cost way less work at least as well.