I've yet to see one that blows down (on an Athlon XP stock cooling unit, at least), but really all you'd have to do to change that would be to flip the fan. All it takes is a screwdriver.
Edit: Here's a link to one on newegg for 3.99. There is also a green one on there (this one is blue).
http://www.newegg.com/Product/Produc...82E16811998131
If you know the size of your CPU cooling fan it would help.. I'm guessing 60mm? That one is for an 80mm.