You can't buy plants unless you buy a tree with plants already growing on it. You can however grow them from seed. Read my blog on it.

http://www.growsonyou.com/fractal/blog/5036-growing-mistletoe

Mistletoe is a parasite that grows on trees (it thrives by inserting its roots into the host's system) which is why you will not find plants for sale. You can buy the berries online from www.mistle.co.uk. They will send a comprehensive booklet with cultural instructions with your order

All you need to know about growing it in my blog too :-)

The link no longer works, but you can pull it up by going to webarchive dot org or wayback machine, both are the same thing. Just copy and paste the URL. Cheers.