Four varieties are considered threatened here in Pennsylvania, including White Lady's Slipper (Cypripedium candidum), Yellow Lady's Slipper (Cypripedium parviflorum), Greater Yellow Lady's Slipper (Cypripedium parviflorum var. pubescens), and Showy Lady's Slipper (Cypripedium reginae). I've heard rumors that some of the latter grow near a local swamp. But nobody posts maps as to their location--for good reason.
One of the main reasons for wild orchids becoming so rare is that people frequently try to transplant them into their gardens. This seldom works, since few of us can provide the forest or swamp conditions the plants prefer.
Because lady's slippers are difficult to propagate, you will have to pay a high price if you purchase nursery-grown plants. I would avoid buying any that seem suspiciously inexpensive, however, as they were probably harvested from the wild. If you can spare the money to purchase nursery-grow lady's slippers, you might want to opt for hybrids, as they are supposed to tolerate regular garden soil better than the species types of lady's slippers do.
For those species types, most growers recommend digging out that garden soil and replacing it with a mix of very fast-draining ingredients such as forest soil, seedling bark, sand, and gravel. You then set the plant on top of that substrate, with its tubers spread out horizontally and covered only with an inch or so of compost. Because most lady's slippers must be kept damp in at least partial shade, they require excellent drainage to prevent their rotting. Two of the cypripediums pictured here, kentuckiense and parviflorum var. pubescens (in the thumbnail), are supposed to be among the easiest to grow.
Although the seeds of lady's slippers aren't expensive, since the plants produce large numbers of them, those seeds require a certain type of fungi to germinate. It is usually recommended that they be sown around the mother plant, which can be tricky if you don't have a mother plant! I did find one seller who offered a little soil from his orchid patch along with the seeds. But I suspect my chances of germinating any are still about as slim as the seedlings would be!
Lady's slippers generally don't do well in warmer climates either, as they require a dormant period during the winter. If you like the look of them, keep in mind that the very similar but much more tropical Paphiopedilum orchid hybrids can be easily grown in pots indoors.
I'll have to ask my aunt whether she remembers where she saw those lady's slippers, since some colonies can last for a very long time. It is my fear, however, that they may be slipping away from us as rapidly as the "greatest generation" is.
So, if I actually locate any, I'll just take their photo and nothing else. There are some plants which I suspect were just never meant to be domesticated!
Photos: Cypripedium acaule photo is by spex, Cypripedium calceolus photo by pascal, and Cypripedium reginae photo by anyjazz65, all courtesy of Flickr Creative Commons. Cypripedium kentuckiense is by an unknown contributor and Cypripedium parviflorum var. pubescens photo by Daniel Layton, both courtesy of Wikimedia Commons.