Reptibark, coco coir or even cypress mulch are good at retaining humidity and can be found inexpensively. You can also make a humid hide out of a tupperware container and damp sphagnum moss. Place it in the cage for them to utilize whenever needed.
I use thermostats from amazon for about $20. Not everyone recommends them, but I havent had any issues with our 3 snakes and tortoise.
You can also get a point and shoot laser thermometer on amazon or walgreens for about $10.
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