Asus still has all their products on their site, e.g. my current card:
http://www.asus.com/Graphics_Cards/MATRIX_5870_P2DIS2GD5/ which I know they haven't made in years (yes, it's very old, I'd love to update it if anyone could get stock of the card I want...grumble grumble...).
You might be better off trying to find a used one, maybe try eBay or Craigslist.