per request of droseraguy....
Directions for hooking up an Old Computer fan. Computer fans are going to be your cheapest fan type available, but you may also find others as surplus.
Computer fans typically run in a few sizes but 80mm or 120mm would be the better sizes for a terrarium. If a fan came from a computer case you'll be working with a DC fan, not an AC one. Depending on the size it will either run 5v or 12v. You'll see a sticker on the fan somewhere that will tell you.
Next thing you'll need in a AC to DC adapter, these are the things you use to charge a shaver with, plug in your cell phone, battery charger, some xmas decoration, etc. You probably have an extra one in a junk drawer that might work. If you look on the sticker of the wall adapater, you'll see an INPUT rating and an OUTPUT rating. You'll want to make sure that the output rating is 5v DC or 12v DC. But there's a fudge factor with this as well, a 4v one or a 8v one might work as well, motors can be forgiving. The important thing to look for though is that it is outputting DC voltage, or AC if you found an AC fan. Not all fan motors are interchangeable from DC to AC. One other rating that is given is amperage you'll probably want around 100mA to 200mA (0.1A or 0.2A) as rating. You'll want to stay away from ones that list something like 1A or 2A. Some cell chargers may be this high and would be too much.
Once you found your parts, you simply cut the end off the fan and the transformer, then strip the ends of the wires. You can use a sharp knife and carefully cut away the insulation if you don't have wire strippers. Next you'll want to find the polarity of each piece. The fan is usually obvious, red positive black negative. However the transformer may be more difficult. The best way is really to just twist the pairs together and plug it in. If it works, you got it, if not switch them around.
*Note: don't let the positive and negative wires of the transformer touch. That creates a short and can burn out the transformer.
Once you get the wires connected correctly, you can just take electrical tape and tape around each pair. Make sure it's nice and secure, you don't want to pull them out by accident.
Then you're done!