There are a couple pressurized singles in addition to the pilatus PC
-12. These include the Socata TBM-700, and the Malibu Mirage. All of these aircraft are powered by turboprop engines, I think all use a derivitive of the PT
Most of the high end singles that are not pressurized come with an oxygen system as standard or optional equipment today. If not, you can buy a portable system that will do the same job. In most cases though, it strikes me as being impractical to fly a piston single that high. Aside from mountainous terrain, you really don't bennefit much by taking the time to climb that high. If you're in a hurry, you might get a better tailwind but sitting up there for hours on end in any single, most of which don't have good cabin heat systems, is not my idea of a good time.