All the other answers are indeed correct. Most pet stores, checking out trader clubs and even facebook and Marktplaats (the Dutch craigslist) can be used to obtain one (or more) of those fuzzy little critters. read more
The only thing I would advice, that if you want to buy one for Ferretting (hunting rabbits using a ferret, which is legal and doesn't require a hunting license, except permission from the landowner) you'd best get one from another ferretteer. They could also give great tips on raising and training them. read more