it's widely available n 10, 20 and 30 thou thicknesses.
Translation
Thou ( thousandths of an inch)
10thou = 0.254mm
20thou = 0.508mm
30thou = 0.762mm
and so on.
Let's take marines as an example. For Pauldrons, torso and legwork, 30thou is generally as low as you'd want to go. For layerwork on top of a 30thou frame, use 10. For rims on top of 30thou, use 20 thou for a defined height difference (shoulder pad trim)
I've used Fils Model supplies for my needs, but can't recomend them as yet until I've made a few orders. Still waiting for my first order after a few teething problems. That said, they are cheap. I can neither recommend, nor advise agaianst them as yet.