Instead of the 9V battery i recommend a USB to 3-pin cable, $5 on amazon or wherever. Works great, i always use three of those each with a 200mm fan in summer. No AC...
I looked up a possible USB option and it seems that they only put out 5 volts. The fans require 12 each so a 9 volt would seem like it'd give them more umph at the cost of having to replace the batteries eventually.