I used to get stuff like that from a local remnants place but now I go to a shop called "Mill-end Fabrics." They get the tag-ends of industrial cloth (from clothing factories, etc.) They can be counted on to have pretty much anything fabric, sewing or upholstery related (including "memory" foam) I need.

Try the yellow pages!

Try Crutchfield for universal speaker mounts. If all else fails the lower front part of the door has nothing in it (Don't for heaven's sake do what my PO did... cut right next to the thigh area of the seat and cut off the lower window regulator mount... the one that holds the lower pulley!).

The fender seal in front of the space you would cut into in the kick-panel is not too good so if you do cut there, seal the speaker in a cup made for the job and seal around the hole real well!

Lots of sound deadening stuff in previous posts.

--
Mike!