Hi Katie,
I suspect that the real articles are much more likely to be found in upmarket stores in the cities of Bourgas or Varna, rather than Sunny Beach, which is solely a tourist resort.
How much cheaper they would be (if at all, as they would be aimed at the relatively few Bulgarians who can be described as 'wealthy') I can't say.
For info: Ladies' shoe sizes (not for your required brands) in Metro Cash & Carry's online catalogue appear to be 25 - 47 - haven't a clue how those equate to UK sizes.
Baldur