That's a good question
Most of the more revealing models are made by independent company’s in very small numbers, often sold at special events in Japan only.
Very often these are model kits, so you don't get the complete figure just the pieces that you have to assemble and paint yourself.
Others are modifications of regular figures, the one in the picture sold for 36,600 Yen today.
If you have a yahoo.jp ID you can find them on the yahoo japan auction site,in the adult section. Google yahoo deputy service for options how to buy from yahoo and other Japanese websites.
There are also some regular figures where you can remove the skirt or the top (cast off).