The supermarkets in the Rila are the best bet. On an English pay and go contract it was about 40p to text the UK. Send one to the UK and ask them to send it around for you. You'll be able to buy a sim that day or next morning.
The instructions for the sim are repeated in English at the end of the BG blurb, so it's easy enough to set up.
other thing we always do is text the hotel phone number and room number back to uk and get people to ring into us, its much cheaper that way - some hotels charge a disgusting rate and you only find out when you check out and get the bill