Depends,you need to shop around is the short answer,sometimes one place has a better deal than another on a specific plant,my personal rule that I follow, (through much trial and error) is simple check any-and everywhere but before you buy check e-bay first you can try cold hardy palms,but also put in the specific palm,C H palms does not always pull up all the C H palms also make sure the seller has been around a while,make sure the plant your getting is as pictured or is well discribed ebay can have some unbelievable deals on the right day,last time I looked they had some smokin deals on trachycarpus fortunei good luck!

If you want large plants, (which are much hardier than small mail order plants) Chilly Palm

I second Chilly Palm Tree Co as it is the best service and delivery. Sean McFall is the owner and he is great to deal with.

Try the Oikos Tree Crops in Kalamazoo Michigan. It's got to be the furthest north that cold hardy palms are propogated out of doors. They are grown in walled in shelters to protect against wind, that's it.

I've ordered several from this company and have had very good luck.

http://www.oikostreecrops.com/store/home.asp

Here is a link that might be useful: Oikos Tree Crops Kalamazoo Michigan

I think I'll give http://seedrack.com/01.html a try. They are just seeds but they are cheaper, and I have some time left to wait for them to grow I suppose. They also have several of the hardier selections. I've been to chillypalm too they look respectable. The thing I'd think about getting plants that have already been growing is you don't really know where they've been growing at. A more northern claim of provenance is probably better. For zone 7 I like several like Windmill Palm, Silver Mazari Palm, Louisiana Sabal Palm and even the Joshua Tree. There might be more thats not coming to mind, not coming to mind because I don't know anymore myself!
Will