Front surface mirrors are considered best because a rear surface mirror can cause undesirable ghosting due to the light path having to go THROUGH the glass for a rear surface mirror. There's at least one ebay vendor with a good reputation and Surplus Shed sometimes has useful mirrors. Some folks have stripped a conventional mirror to expose the silvering and converted a rear surface mirror to a front surface mirror. A little creative searching on these forums will reveal more detail.
Edit: Check my ploD for a 3 point mount idea. I personally don't recommend mounting the mirror ON something because even slight deformation can cause projection artifacts. You want a totally planar reflective surface.