I went to one of the local glass dealers and asked for pieces of mirrors that they could donate. I bought a glass cutter at Wal-Mart and cut the mirrors into small 8x10 pieces and taped the edges. My students hold the mirrors when drawing.
If you need for them to stand up...get some pieces of 2x6" scraps of wood...have someone handy with a saw cut a groove thick as the thickness of the mirror. It should also be cut at an angle.
I ask because:
1. I inherited my art room without any mirrors, and
2. The cost of buying decent acrylic mirrors made for art classes would
wipe out a substantial part of my budget, and
3. I saw some approx. 6" diameter round mirrors at the dollar store
today, that have an adjustable angle and can be propped on a table.