Same as during the day - you only need to change them as part of a breeding cycle.
Where are you located and maybe some members can recomend a store near you?
dr del
Derek
7 adult Royals (2.5), 1.0 COS Pastel, 1.0 Enchi, 1.1 Lesser platty Royal python, 1.1 Black pastel Royal python, 0.1 Blue eyed leucistic ( Super lesser), 0.1 Piebald Royal python, 1.0 Sinaloan milk snake 1.0 crested gecko and 1 bad case of ETS. no wife, no surprise.
You can buy an excellent Herpstat thermostat at the wheaton, IL reptile show/animal expo. There's a vendor there every show and he almost always has brand new herpstats. He's an authorized dealer so you get the full warranty and everything.
allanimalexpo.com is the show website.
Barring that, you can sometimes get a ZooMed 500R at Petco or PetSmart, but you will pay $40 or more plus tax when they can be bought online for about $22 plus shipping.