I purchased 2 mirrors at Target for $2 each. They're about 5" by 7". They were in magnetic frames and I found them on the school/office supply aisle. I tried the Dollar Store, but the manager there told me they only carry that stuff during back-to-school. They're marketed as locker mirrors, despite the fact that lockers aren't used anymore. At least, not in my area of the planet.
Anyway - I plan on duct-taping the mirrors together, but for now just holding them in the right spot works.