I have looked on the post office website and they advise taking US dollar's to bulgaria, but I have seen people saying that sterling is the thing to have.
I took US dollars last year ,only becos i had them left over from a trip to the states, you can change them anywhere , changed mine at the samokov reception , however i dont think you get any advantage using dollars, just same sort of exchange rate as you would if you used sterling straight to levs.
Kel
Unless you already have them, don't bring dollars, the rate is rubbish, my son brought them from tiawan at christmas and really got a bad deal, lost a couple of hundred quid on the various exchange rates