Unless you can speak fluent Bulgarian, why risk getting a local to buy it for you. Whilst it may be cheaper for locals to buy their own lift passes, is it worth taking the risk for the sake of a few pounds. There's very good reason the locals get cheaper lift passes - what you probably earn in a month, they earn in a year...
Don't be a skin flint, do like everyone else, get your liftpass before you go.
Where do you get them before you go...how much do you save ...and might it just be worth seeing if it has snowed when we get there?..... toooooo many Q's!!