Usually customers book tickets from transport company offices, not from the bus stations. Online sales or booking offers are rare, especially for internal destinations. Most of the offered online tickets are for destinations in Europe.
Information provided at the bus stations websites is often in Bulgarian only.
Therefore foreign customers can use the services of tourist agents, hotels, hostel or tourist centres, which can help with booking and buying tickets for foreign tourists or residents in Bulgaria.
Ministry of Transport
Wikitravel – Bulgaria