The main Malina (2) have upgraded their skis recently and now offer good quality carvers for all.
An alternative would be to try Pete Buggeroff (nickname) at Interski also hires good quality carvers. He can be found in his shop just below the Markony Pub and next to the Snowshack.
Ski Elite facing the main ski shed is also a good bet, no queues, small family run business, quality equipment, very hospitable people and good food and hot drinks when you leave back your skis in the evening.
You could try Tanya Ivanova just back from the ski station 1 lift. You'll see the big red Rossignol signs and that what she hires. Some good gear in there.
Pete at Interski offers a good discount to Bulgariaski forum members. Echo the comments about excellent tuition too - and cracking hospitality!
End of first day, Pete ends the days with " wine or whisky??" - now you don't get that in many ski hire shops...