Yes but it is not necessarily cheaper
There are very few shops in Tulamben, the Wayan supermarket may have SIM's but I have never seen them there (we spend 2 months a year in Tulamben and are here right now). And I don't remember seeing a pulsa sign at any of the small warungs either. We always recharge enough before we arrive to see us through our time here.
If Wayan's can't help you, ask at your hotel. One of the staff could probably get one, even if they had to go into Kubu.
Unless you are coming way past 9pm. You should be able to pick up sim anywhere in between while you are travelling to Tulamben -It's a long journey of 3.5 to 4 hours.