If you've run out of bat detectors but the bat detector store has already closed for the day, fret not! Here is kryptonaut's report about constructing a bat detector with a Raspberry Pi and other inexpensive components. The code isn't made available, but if you have some experience coding bare-metal on the Pi it shouldn't drive you too batty figuring it out. Click through for a recording made with the device.
It's basically a kind of granulator which slows down incoming audio by up to 4 octaves, but works in approximately real-time by overlapping the slowed down sounds as necessary. I originally designed it for listening to slowed down birdsong, but the mics I used are responsive up to 100KHz so it's turned out to be a rather effective bat detector – it's very cool to hear them chirping as they fly overhead at dusk.