McMaster Carr -- very well organized online catalog, but sometimes more expensive. Extremely fast
shipping (here in
California orders sometimes arrive the same day).
MSC Direct -- often the least expensive for random industrial
parts, fast
shipping, they often have sales with 20-50% off on huge
classes of things ("safety equipment" "metal cutting tools"...), so a very good deal if you hit one of those sales.
Grainger
I've used the 1" lead lined foam with the
aluminum foil face, both on a
boat and when building enclosures for machines. It works very, very well, but is also one of the most expensive options. So I am also interested to hear people's experiences with less expensive materials.