The only rear view mirror that was available as standard equipment in 1931 was the inside rear view mirror. Trico sidemount mirrors were listed as a genuine Chevrolet accessory in the parts book but they are very expensive if you find them today....and you must have sidemounts to use them.
Aftermarket door hinge mirrors and the like are available from various suppliers like the Filling Station as you already experienced. If you want an outside mirror one of the mirrors from the Filling Station would be the way to go.
