The genus Paphiopedilum comes from all over Asia, ranging from China to India in the west, and Indonesia to the east, and many islands in-between.  The genus Phragmipedium comes from Central and South America.  Then there are Cypriprediums which grow in North America.  (And two other genera, Mexipedium of which there is only one representative and Selenipedium of which there are around six species).

There are about 80 or so naturally-occurring species of Paphiopedilum and 25+ species of Phragmipedium.  Species plants sold in the US are almost all artificially propagated, or divisions (i.e., new shoots grown from an existing mother plant).  Paphiopedilum and Phragmipedium plants harvested from the wild are not legal to own in the US, unless they were obtained prior to the 1970s when the US became a signatory to a treaty barring trade in certain species of animals and plants. 

Man-made hybrids made by crossing different species with each other number in the thousands.  The diversity of form and color is simply astounding.  Typically at orchid shows, newcomers to slippers are most impressed not so much by the natural species, but by the amazing shapes, colors, and patterns found in many hybrids.  Some people actually find it difficult to believe that these are indeed living plants.