On Amazon the thermometers look like they go from $5 - $10. If you're already getting something from Amazon so that your total is over $25 and you get free shipping it wouldn't be too bad. It seems like I've seen those things in the grocery store in the isle where they have a few pans and other cooking utensils.
An important thing about freezers is to make sure you don't pack them completely full. Leave about an inch all the way around if you can, failing that never leave less than an inch on the back and top. If your freezer has vents on the sides instead of the back, then leave the room on the sides and top instead of the back. There needs to be room for the cold air to circulate.
Also with the fridge, see if you have a duct somewhere in the back on the ceiling of the fridge, and try to keep things away from that duct. For one things will freeze next to that duct most times, and for two you don't want to impede the airflow there either.
When you close the door to the fridge or freezer, don't just swing it and assume it closes. I like to hold my finger on the rubber seal while I close the door so I can feel the seal slowly compress as the freezer or fridge vacuums closed. That way I always know my fridge/freezer is closed.
