I personally like the Sunbrella curtains and have them on my carts. I used Eugene at:
Eugene's Quality Golf Cart Enclosures
a couple of years ago and was very pleased. My father had two carts done last year.
You would expect to pay about $650 for side/back curtains, $100/seat and optionally $100 for roof cap.
An old thread exists here with some other vendors and other recommenadtions...and how mine turned out.
Enclosure for Club Car