I have been looking for products that are made with acoustic foam to isolate noise in the trunk and behind the seats.

I've read posts that recommend this, but caution that the thickness can not exceed 1/4 inch or else the panels won't clip down.

I have contacted jcwhitney, earsc.com, and soundabsorber.com as some have recommended. They don't carry acoustic foam in such thin thicknesses.

What have other people used? Where can I get it?

 

You're going to have trouble finding quality foam pieces that thin for a low price. The cheaper it is, the thicker it needs to be to absorb the noise.