Check your local salvage yards. Not sure if the FiST bumper is totally different than the base model, or if the ST rear bumper skin just has add-on pieces. Salvage yard might be able to tell you, or you could look up the parts online.
The normal Fiesta rear bumper isn't the same, I'm actually in the middle of replacing my rear bumper as well, someone backed into me
1800miles on the car.
The FiST has notches to accommodate the small rear skirt vs the normal Fiesta doesn't have these, you want the following bumper, part #:
D2BZ17906AB
Hope that helps, to give you an idea, in CAD, the rear bumper was $722.37 [emoji20]