Using your location...
http://www.walmart.ca/en/ip/bios-digital-inout-thermometer/6000075838818
That's available at your walmart off of Quinn. Actually a decent thermometer for our keeping needs. You can use adhesive industrial velcro (found over in the fabric and sewing department) to mount the thermometer in the cage. Mount the main unit at middle height on the back wall of the cool side of the cage. Then run the wired probe (aka outdoor probe) to the middle of the under tank heat pad. This will allow you to monitor both cool and hot side temps.
Secure in place with hot glue, aquarium silicone, or duct tape. If you use tape, make sure the snake can't get under it and thus get stuck to the tape. Hot glue and aquarium silicone will obviously have drying/curing times and will be permanent for the most part.
WARNING: duct tape's adhesive tends to get a tad viscous/fluid when warmed and eventually dries out. A better option would be waterproof aluminum ducting tape. However, this tape's adhesive properties make traditional duct tape look like weak clear office tape.
BE VERY CAREFUL IF YOU CHOOSE TO USE WATERPROOF ALUMINUM TAPE...OR ANY STRONGLY ADHESIVE TAPE.