I did a search for acoustic mat and didn't find much. I thought I'd at least post where you can get it. I'm going to get some for my car as soon as I can crawl in there and see how much I'll need.

This is basically all Jcwhitney has:
Jcwhitney.com - 32" x 54", 1/16" thick, (1), $30
Jcwhitney.com - 10" x 13", (12), $40

Summit Racing has a lot more than this:
Summitracing.com - Dynamat, 4sq ft, .070" thick, $23
Summitracing.com - Dynamat, 13sq ft, .070" thick, $70
Summitracing.com - 18" x 32", .059" thick, $18
Summitracing.com - 32" x 54", .059" thick, $50
Summitracing.com - trunk kit, $140

This is all Crutchfield has:
Crutchfield.com - trunk kit, $130

Parts Express has more than this:
Partsexpress.com - 10" x 13", .050" thick, vinyl, (1), $3.85
Partsexpress.com - 27" x 40", .050" thick, vinyl, (1), $26.30