LocostUSA.com

Fabrics:
http://www.garysupholstery.com/products.html

Foam:
http://www.garysupholstery.com/upholstery-foam.html

Foam stocked by fabric shops isn't stiff enough. It needs to be 2 inches thick for seat backs and 3 inches thick for seat bottoms using their low density foam.

Automotive and motorcycle seats use much denser foam.

Foam is not cheap, so cutting up a donor vehicle seat to get the foam is worthwhile.

A hacksaw blade or serrated kitchen knife work well if sawing back and forth a lot with little pressure on the foam to prevent tearing.

The foam is smoothed out slowly with a belt sander and light pressure. It is easy for the sander to grab the foam and dig into it, creating a low spot.

Hog rings are loops of metal that hold the upholstery to the seat. They are soft metal and easily reusable after straightening. Needle nose pilers work well. They do not have to be cut off.

Sometimes, straight wires are embedded in the foam where the seat is recessed in the middle, to hold the upholstery in this low area. If you don't have to remove the wires, then don't cut them out. The donor seat is free, so you might as well experiment.

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Miata UBJ: ES-2074R('70s maz pickup)
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LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12