Plant Delights in NC has several lady slippers, but they are very pricey. Are you sure you have a good site for them? I would hate to buy and then have them die. Yellow ones are the easiest here in the Piedmont.

Shooting Star Nursery, Frankfort, KY offers 4 species of LS at reasonable prices. Small, I'm sure, but if you want a number of species/plants and can provide suitable growing conditions, it is probably the best way to go.
All of the ones they offer, should do well in your area and are relatively easily grown.
Rb

Here is a link that might be useful: Lady Slippers

Here are few more good sources:

http://www.orchidmix.com/cyps.htm
http://www.wildorchidcompany.com/0607/Hardy%20Orchid%20Price%20List0607.html

The second one (Wild Orchid Company) is in PA. If they are near you, it might be worth a visit.

Not to beat a dead horse, but as the previous posters have said, a good site with suitable growing conditions is very important. They are not as difficult as many of the myths out there, but the growing conditions must be right or they will not survive beyond a year or two.

Here is a link that might be useful: Spangle Creek Labs

Loved the websites, but the prices listed were still too high for me to want to experiment. Anything over $25 would have to be a Christmas present! I have pink lady slippers here naturally and have had some long-term success in re-establishing them after rescues. My success with the yellows has only been moderate as they seem to need more moisture than we have had the last few years.

RB, watch shooting star, I think they are just reselling Spangle Creek's out of flask seedlings at higher prices.

Northeastwisc has probably listed the two very best sources for established plants.

There are some old threads floating around this forum that list just about every source there is.