As of 2018, there are NO large public buses to Bukit Lawang anymore, instead, smaller orange minivans travel the route every 15 minutes. (These are refereed to as the public bus).
Get a Grab taxi (40,000rp from city centre) to drop you off at the Mawar Bakery & Cake Shop which is 1km along the road from Pinang Baris terminal. This is where the orange “FA.PEMB.SEMESTA” Bukit Lawang buses leave from. The actual fare is 35,000rp per person but expect to have to haggle down for this price.
Do not get dropped off at Pinang Baris bus terminal!, scammers are waiting there and will quote 120,000rp for a bus to Bukit Lawang. They will follow you the 1km walk to the bakery and talk the driver into charging you an extra “commission fee” to go into their pocket. Avoid this hassle and go to the bakery directly, find the orange buses parked along the road & try to find the bus driver, and avoid the scammers.
The last stop of the public bus, where you get off, is normally the bus terminal in Gotong Royong, a village just 5 minutes from Bukit Lawang. From here you can get a becak (motorbike/sidecar/tuktuk type thing) for 15,000 IDR. This should drop you at the national park office in Bukit Lawang and you can then find your guest house.
There are many drivers offering minivans from the Belawan ferry terminal direct to Bukit Lawang, bargain hard. Expect to be shifted minivan at least once (in Medan). Prices vary between Rp. 60,000 and 70,000 all the way to Bukit Lawang. The extra cost should outweigh the hassle of getting into Medan, finding the bus station, then finding a hostel in Bukit Lawang (especially if you arrive during a torrential downpour). The driver will usually recommend a family or friend’s hostel.
You can go by tourist bus (minivan 5-7 persons;) from Parapat (Lake Toba) or Berastagi to Bukit Lawang. Tickets can be bought at local tour agencies; but sometimes they run the vehicle as speed driver; to reach the arrival place; just check it out on their sites