there is no reason why you couldnt run more than one fan if your adapter is rated for enough amperage, most adapters are rated in milliamps and the pc fan you provided a link to on ebay says .6 amps wich is equivelent to 600 milliamps per fan.
if you dont have one that you can use already laying around there is several places that sell universal adapters, or you could even remove a power supply from an old computer.