Sparks sells a Cazio seat COVER, which is basically a high-end slip cover.
I've always been impressed with Katzkins. They have an excellent rep, make quality actual seat material replacements, and I am told now have a HUGE variety of colors and textures.
I have a set on my Miata that were installed in Winter of 2001-2002. They are abused (VERY seldom cleaned or ANYTHING, in a covertible that is usually top down and in sun and sometimes in rain), dirty, and well-used, but are STILL soft, supple, and look GREAT!
I have been impressed with them.