I'm a cider drinker too Callum but normally have to find an alternative while I'm in Borovets as it's expensive compared to everything else. Bobbys bar sell Strongbow by the bottle. A couple of the shops sell Magners for about 6 or 7 levs a bottle. The Iglika hotel sells Magners in the lobby bar but I can't comment on any other hotels as that's the only one I've stayed in. All imported drinks cost more than the local ones.
I am a Strongbow drinker and last year found a bar (opposite the Mucky Duck) which sold Strongbow on draught. A pint of Strongbow and large white wine 10 leva. (Less than £5.00)