I don't think the convex schoolbus mirror is what FarmwithJunk was think of, or CCAP74, but probably that large flat mirror just above the drivers head that he used to "watch us" kids with. Probably about 30" long, and six inches high (rough estimate from about 35 years ago experience)
If I were looking for one, I would follow the yellow brick road. Chase down where the local school buses go once they have graduated from being school buses and head for the big bus parking lot in the sky...
Used parts have to be available somewhere!
David from jax