We do a great business in mirrors. Sell them just like any other frame order. Great excuse to stack frames and sell those gigantic moulding that no one ever buys.
Find yourself a good supplier for mirror both beveled and plate (no bevel) and sell, sell, sell. Beveled mirrors come in a wide variety of standard sizes and shapes. You can also have them custom made, but they get a little pricey.
I would recommend selling nothing but 1/4" plate or 1 1/2" beveled for anything custom. They are the better quality. You can use the 1/8 (shock) mirror on anything smaller than 22x28, for a less expensive option. I would not suggest it for larger mirrors unless you are specificaly looking for the "Fun House" effect.