hehe, I'm talking about random AC adapters like that are used for electronic devices such as phone chargers, or keyboards, or printers, etc.
From what I know, as long as the voltage of the adapter isn't more than the voltage of the fan, you're good. So as most fans are 12volt, you can use adapters that are 12v on down, but the lower the voltage of the adapter the less power the fan gets and hence the slower it'll spin.
So you cut the end off of an AC adapter, and the end off of a fan wire. Then tie the positives together and negatives together, make sure it runs, then use electrical tape to tape everything up. Then you're good.
Hopefully.