During the peak season of July and August, it’s not a bad idea to book ahead. From October to May, it’s a good idea to ring ahead to make sure the camp is open and serves food. A few places stay open in winter.
Apart from ger camps, many individual families rent out a spare ger (and/or hire horses), normally at cheaper rates than the ger camps. You’ll have to ask around as none advertise. Expect to pay around T10,000 per person per bed.
Unless you hike out into the hills, it’s best to get permission for camping, either from the nearest ger or, for a fee, a ger camp. Pitch your tent away from the main road, don’t use wood fires and take all of your rubbish out.