There are a couple of options as steel return rollers go. Tank Work Shop makes a set along with the shot deflector for the commander's cupola for your early to late conversion. There is also a set of white metal return rollers made by Model Kasten. The only place I have seen them was in Osprey's Modelling the Stug III. Then the final option is using the ones from a DML Stug III Ausf G, however the "axle" between the two halves is to wide and needs to be narrowed for a correct width return roller.
TWS 2060 STUG III Detail Set w/Late Mantlet
As for the Model Kasten ones I am not sure where to find them. I looked on several online stores in the U.S. with no luck. I hope this helped.