Sofia is the capital city of Bulgaria lying in the west of the country.
East towards Plovdiv
Take the bus number 5 and go with it until it enters the highway. Along the highway there are lots of tankstations, but it is recommended to go with the bus until it leaves the highway out of Sofia. There ask the bus driver to get out of the bus, otherwise you will land in the little village of Lozen in the middle of a beautiful nowhere. You will stand on a motorway junction with directions North, East to Plovdiv and West to Sofia. Go down the ramp to direction Plovdiv and you will see some shops where people stop next to the highway. Try standing on the hard shoulder to get a lift or ask people there. Nearly all traffic there goes to Plovdiv or even further east.