I'd go for the two fans in series suggestion, but you will want to get fans that draw the same current (or very close), since the same amount of current will be going through each. Dead PC power supplies are plentiful, and I've yet to see a non-working fan in one. The higher end ones even have two fans. I have a number of them (the fans that is) in my junk box.
"A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979