It leaves from a bus stop on the south side of Peace Avenue, opposite of the big 'Narantuul Hotel', 150 meters East of the junction with Ondor Gegeen Zanabazaryn Gudamj, which is the road that runs south from Gandan Khiid. The bus waits at the east end of the bus stop. The bus stand has a large "Pepsi" sign on it (GPS N47.91545 E106.89782).
Buses leave at 16:00pm, and take 2 1/2 hours to get to Terelj village.
Coming back, they leave Terelj village at the uncivilised hours of 08:00am and 19:00pm. The bus costs 2200 MNT one way (as of May 2013).
Ecotourism Ger Camp, (You can phone Bert for a pick up from the bus, or directions are to turn right after the UB2 hotel and wade across the river. After that turn right (east) along the track alongside the river that follows the electricity poles. When you reach pole numbered 40 turn 90 degrees right and it's in front of you.), ☎ 99734710. The affable Bert and his family run a friendly ger camp away from the main tourist area. He can arrange horse riding further into the park, including Gunjiin Sum and there's good hiking in the area. The food's great (including home made cheese).20 EUR Per person per night including all meals. edit
Other Tourist Ger Camps. There are numerous tourist Ger camps on the road as you approach Terelj village, some are closer to the road than others. Any of them can set you up with horse rental, otherwise you can just hike around the park.25,000 Tk for a 2-person Ger (no meals included), you might be able to bargain a bit, since there are a bunch of tourist camps and they're fairly indistinguishable. edit
Chinggis Tourist Camp. Traditional tourist camp. Provides accommodation in the round felt traditional houses (authentic Mongolian gers) used by nomads, western-style amenities, including a sink, toilet, twin beds and electricity. Western-style showers, bathroom facilities, and restaurant facilities. There six gers with two beds, seven gers with three beds. The gers are decorated with traditional colorful patterns and furnished with soft and cozy beds. Each ger is furnished with beds, blankets, bedding sheets, pillows, small towels, bath towels, a table, stools, candles and matches, thermos, glasses, etc.
Authentic Mongolian Ger Heating. Heated by the wood stove placed in the center of it. The ger keepers start fire for you and can keep setting fire from time to time during colder nights.