When my wife saw how serious I was about learning to sew upholstery materials, she hit Joanne's to find me a bunch of thick materials to practice on. She rummaged through the remnants and came across some of that Whisper vinyl, and brought it home along with a lot of other material. (The back of this stuff looks rather "furry" for lack of a better word - not a tightly woven cloth backing like other automotive/ marine grade vinyls I've seen.)
I've done some upholstery in the past, although I've never sewn before, and I know this stuff is pretty thin. I'm going to use the piece she brought me home to make a sewing machine cover.
Vince is right - this stuff won't wear well at all. Especially for something that's going to get a lot of wear like seats. It might work pretty well for door panels and rear shelves (package trays,) but other than that, I'd steer clear of it for automotive use.
We have a place here locally called Mill End Fabrics. They have a lot of upholstery materials, to include vinyl, foam, and batting, but like any other retail outlet, you're going to pay a retail price. That might be okay if you're just doing a seat or two, but it can get pricy real quick. They want $11.99 a yard for the marine grade vinyl I need to do my boat. That's actually not bad when you add in the fact that I won't be paying for any shipping, but as you can see it's going to add up pretty fast.
Just my 2 cents, and remember that I'm not a professional.