I think this is what you are seeking. They hold the hub ('39-'48), to the backing plate in case of an axle break. If so, just about any speed shop will have them. Speedway would be a good start.
They're very similar to the car ones, but the vertical lip that the retainer sits over is not vertical, it's on enough of an angle on the outside edge that the retainer will neither fit nor have any grip if the axle breaks.