Where could I get folded canvas roofs for the tamiya Horch 1a or even generic folded roofs that would fit this vehicle.

If you can't get the part from Tamiya, consider using brass or aluminum rod as an armature and making a replacemenmt from tissue paper and thinned white glue. You'll have nice texture and folds.

Im sure if you ask in the buy, sell and trade forum you should be able to get one. Some kits come with both options ... up and down ... so if anybody has built the hood up version they will have the hood down as spare!!