To answer the question "Where to Buy a Rear Subframe" the correct answer simply is the place that has the CHEAPEST GENUINE SUBFRAME of the type required in stock. Try Google, you may find one cheaper at a different supplier.
It is not the supplier who you should be concerned with, except as regards profit margin. It is only the MANUFACTURER, and only Heritage frames are up to scratch at this point in time. The pattern parts almost all all need additional welding for safety as they have been skimped, and most need serious attention to the alignment as the pivot pin holes are not drilled accurately, amongst other problems.