Check this place out:
http://www.gahh.com/side1.htm
They have color samples on line - and complete seat kits for 500E's. I just had a carpet repair done in my wife's 300TE and the upholsterer used GAHH material. The material looks good, and the color matched the 300TE's unusual creme beige color. I'm not sure if they have pre-cut pieces - my upholsterer just bought the material.