You can make one - cut balsa wood (available at a craft store) to the size/shape that you want, go to Walmart or your local Goodwill store and buy the cheapest unframed mirrors you can find, cut them with a glass cutter to size(can get one at Hobby Lobby or Michael's, and probably even someplace like Lowe's); glue the mirrors to the wood, glue the wood together, leaving the bottom open.

Only sticking point would be the top, you can get around this by cutting your wood piece with the "slot" down the middle, trace the outline and the "slot" onto tracing paper, cut the shape and slot out, cut the tracing paper in half along the middle slot, and use that as your template for cutting two halves of your mirror top. Easier than trying to cut a slot in the middle of a mirror, which would likely leave you bleeding or uttering things you normally don't under your breath or quite loudly. Neither of which is preferable or necessary.