FCRC Machine lists rubber rear lower control arm bushings for 85-93 Mustangs on their website. They are Rare Parts P#22033. They cost $180 per control arm and include the oval and round bushings. You can also try MPS Auto Salvage in GA; they might have OEM LCAs in better condition than yours.
The poly bushings designed to fit in the OEM shells on the OEM LCAs are not ideal for performance driving situations, because they bind when the suspension articulates. Many inexpensive rear control arms come with 2-piece poly bushings, and they aren't much better. If you are going with poly, 3-piece is always best. Most of the top-tier LCAs have a 3-piece poly bushing option.
But, as other people have mentioned, it all depends on what you plan to do with the car.
The poly bushings designed to fit in the OEM shells on the OEM LCAs are not ideal for performance driving situations, because they bind when the suspension articulates. Many inexpensive rear control arms come with 2-piece poly bushings, and they aren't much better. If you are going with poly, 3-piece is always best. Most of the top-tier LCAs have a 3-piece poly bushing option.
But, as other people have mentioned, it all depends on what you plan to do with the car.