Here are some battery basics that might make it easier and more effective to match a fan and battery. Fist of all the voltage. AA and AAA are 1.2 volts. So if you use a 12v fan you would need to add 10 AA batteries together to get 12 volts. That would be a bad choice (heavy). a "9 volt" battery is 9.6 volts. find a fan that you may want to use and look at the voltage requirements and MaH or AH. (Milliamp hours / amp hours.)
Think of the battery as a motor with a horse power rating and a fuel tank. Amps = horsepower and Mah= size of fuel tank. so a 9v fan with a 1000 Mah rating will draw 1000 Ma (1 amp) at 9V. If your 9v battery has a 10000 MaH rating, you can see that your fan will work for 10 hours. (1000 Ma draw per hour.)
Short and sweet match the voltage for the fan and battery and look at the power consumtion to see how long it will work.
To increase your MaH you can combine batteries + to + and - to -.
To increase your Voltage you can combine batteries + to - and switch the final temination.
here is a link to a helpful web page.
http://www.allaboutcircuits.com/vol_1/chpt_11/5.html
You could aslo use a variable switch and be able to speed up and slow down the fan with a dial.
I know this may be confusing but I tried to keep somewhat short, PM me if you have more questions about batteries (I build my own packs for remote controle planes).