Most people come to Rewalsar because they wound up in Mandi on their way to or from somewhere else. There are regular buses from the Mandi bus stand, or you can take a taxi, which should cost about Rs. 600. Travel to Rewalsar from Mandi should take about 1-1.5 hours.
From Dharamsala, there is also a bus; this takes 6-9 hours, depending on how many changes must be made. It is also possible to hire a car from Dharamsala - the cost is currently Rs. 2650.