aquabid.com, plantedaquariums.com, aquariacentral.com (usually someone has them for sale on there), ebay.com, bluecrayfish.com (user badflash breeds them) and crayfishmates.com. And increasingly enough, one of Petco's and Petsmart's wholesalers have these mixed in with regular feeders... so if you're good with IDs you can pick them out of feeder tanks very easily.
Don't use marbledcrayfish.com though, unless his prices have changed it's excessive shipping charges and he promotes misinformation about the species.