Most all pet stores. Online, amazon too. In case you are looking for CHE it stands for Ceramic Heat Emitter, just incase you didn't know. They last a very long time, like years, so they are well worth the money.
i second the suggestion for using a CHE. gives your tortoise warmth at night while still allowing it to be dark. it is what i use. i ordered from amazon
2 weeks ago I tried using a black light for heating a small sulcata enclosure at night. The tortoise was noticeably disturbed by it and would wake up and move away from that area (even though it was warmer). I Switched to a CHE (purchased at Petco) and that same animal never seemed bothered by it.
+1 on the ceramic heat emitter.
--Jeff
I use a 75 watt CHE for a 40 gal tank. I only need it at night and it gives me 90 under the che and 80 in the corners. It is about 18" above the substrate. But outside temperatures like the room you keep the tank in also affect the temperature ranges inside the tank. If you got both i suggest using the MVB during the day and using the CHE at night
The wattage you need depends upon your enclosure and your home's room temps. I like to use 60s where I can. If that is not enough, I'll bump up to a one hundred watt CHE.
Go back to the pet store and get a thermostat for your CHE. THis will maintain your ambient day and night, regardless of the room temp or any other lights. If your basking lamps are doing an adequate job of warming the enclosure, or if its a hot summer day, then your thermostat will just keep the CHE switched off. If the enclosure gets too cool at any time, the thermostat will kick the heat on.
I like to put my MVBs on a timer for around 12 hours. Adjust the height of the bulb to get the right basking temp.
If you put it on a dimmer or heat controller (CHE), it basically becomes adjustable which for me makes them 10x more practical. They can go in smaller places, you can get "just a little" heat on nights that are borderline. They work great for me.