Edmund Scientific has a 3" F5.8 mirror. Something on that size (like most mirrors that amateurs want) generally needs to be made by the user of the mirror. A 3" size would be made in short order and for something like that, you could get plate glass from 3/8" or 1/2" plate glass from any glass sales shop. Have them cut the glass into a circle and smooth the edges (so you don't cut yourself) and ask them to anneal the glass. Then enjoy the simple pleasure of grinding and polishing your own mirror.