The fan also gives the plants the opportunity to cool them selves if the terraruium should get over heated a little from incoming light, provided you keep the plants sufficiently watered, so they have moisture reserves to expire thru their leaves. You may want to go to a Radio Shack and see how small their fans are. I am sure you can also get the appropriate tranformer to drive the fan too. Personally, I prefer ordering from
http://www.surplusshed.com/ , were they have cheap small fans and transformers, with a flat $5 shipping charge. You might want to get a few of their $1.25 loupes,
http://www.surplusshed.com/pages/item/l1859d.html , while you are at it. These are great for seeing mites on your plants. Saves you from having to treat all your plants for spider mites, as they are very particular as to which plants they like.