Where can I get...

[Tennant]

... the black/grey foam, commonly used to line Rack/Head cases.

My band and I built a drum hardware box and we want to line it with some of this stuff... but couldn't find any at Rona or Home Depot.

What kind of shops sell this stuff?

[Son of Magni]

I'd try a large hobby/crafts type store.

[myles]

if you have a Micheal's craft store near you, they ussaly got alot of stuff there too