12V/0.3A means that it requires 12V and .03A

The fan manufacturer has a datasheet out there somewhere that will have the tolerances of the device (i.e max & min levels etc.)

But for practical usage any 12v source, be it a battery or an ac/dc adaptor, that can supply at least .3 Amps should power your fan.

here ya go:

https://www.youtube.com/watch?v=evQtPGJe7xM

12v wall wart ac/dc adaptor... splice n' dice