Ross has been out of the business of making steering boxes for a long time. Parker bought the company decades ago. The remainder manufactures hydraulic motors and hydrostatic steering valves. The plant was in Greenville, TN last I knew. I suspect you will have to find a used box and scrap it out for parts.
If you go into a good bearing supply house with accurate measurements as to ID, OD and thickness, most will be able to find something that will work. Take the remains of what you have with you. The bearing numbers should be on the inner and outer races.