That really depends on how far back you want to go historically. In the late 600's modern day Bulgaria began as the First Bulgarian Empire, in which people are known to have worn caftans (long coats that you can wear on horseback), wool and linen clothing, leather boots, and decorated belts. Later on historically the region was controlled by the Ottoman Empire, so the clothing likely reflected Ottoman tastes.

If you're looking for a simple answer then I'd go with a caftan, long wool pants (tied at the bottom), a linen shirt, leather boots with flat soles, and a decorated belt.