........ I need the support for my back the seat only sits so strait and it's not strait enough. I'm a tiny thing driving a jacked up '93 Blazer and need the height so I'm not sure what kind of fabric to even. I'll have to buy something because I don't have any old shirts or even sheets to use.
Kittaly, if you plan on buying your material, I'd try the local Goodwill and Sal's first... those tees with ads on the front are usually only a dollar.. and go to the big guys section... the more xxx's the better, you'll get more fabric!
Sheets and curtains can be found there pretty cheap, too, be sure to check both sections. Even a single strand of bright orange/aqua/red yarn worked in with a pale washed out sheet will look good when it's crocheted into a rug... or in you case, a seat mat.
Ask all of your friends, everybody had an old tee they never wear, just a couple from here and there will add some variation in your colors and it's surprising just how far you can make that one tee shirt go.
Beware, rugmaking is addicting... I have them all over the house, some are 20 years old and are still going strong, even being tossed in the washer and dryer every month.