I would get a few levs before you go (maybe £50 worth) just so that you have some Bulgarian currency with you in case you need something before you can change your money.
You can also withdraw from the ATMs. There is one outside T-Market in Samokov which is 10 minutes drive from Borovets. I'm not sure if there are any in Borovets, maybe in the hotels.
There are several in Borovets and it is cheaper to change money this way. You hear horror stories about them never working, but that's pretty rare. You can find them in the Rila Hotel (near reception desk), just by the Green King (near gondola) in the reception to the Flora Hotel and also in the Ela hotel.