The best place and value will be from the guy that sells them on eBay. Not sure of his name, but if you do a search for it (probably on ReefCentral) you should be able to get the information to order them. I think you get a dozen for around $19 including shipping.
The eBay guy is user "721", email is [email protected]. I just bought some plants from him. Was $17.99 for ten 8-12" plants including shipping to Kirkland. Talk on Reef Central is that he sprouts the seeds in freshwater, so you might want to acclimate them slowly. I didn't, and the leaves have fallen off, but I'm told they'll recover in a month or two. -Larry