Behringer sells a stomp box called the ADI-21, which is essentially an acoustic preamp in stompbox form, for something like 30 bucks. I have one and it makes a HUGE difference with my acoustic-electric instruments. Yes, it's made of plastic and the circuit is based on the Tech 21 preamp, but it's a great affordable box.
I prefer that to having to reach into a soundhole to change batteries, as you have to do with some active pups.