Don' know about a part number (the auction has one), but here you go:
SPAL 5.2" Paddle Blade Push 12V Fan, Motorcycle & ATV:eBay Motors (item 200366867993 end time Aug-24-09 15:41:38 PDT)
I think these are the ones that people have been using and that it is also the cheapest you'll find them. Somebody correct me if I am wrong on this.
I was going to order one, after I get around to making my scoops.....
FWIW, I have a switch set up to turn on the stock fans, whenever I want and it works pretty good, up until it's about 95-100 degrees outside. Then it starts getting a little scary when it never drops below 205 on the freeway (15/40 with no pc and slip-ons), then immediately starts going up unless the fans are running 100% of the time while below 40-50mph. I'm thinking scoops, returning to 16/40, ditching the stock fans for one spal, and adding a powercommander will fix it just fine.