A few more scattered thoughts:
Figure #3 here
is a perfect example of the transistor circuit you'll need. Replace R1 with the moisture sensor, and the other side of the sensor (the one not connected to the base of the transistor) will actually be connected to the DC ground (not 12VDC as I earlier said). Get two of those transistors - if you need even more sensitivity, you can connect the transistors as a Darlington pair for higher amplification.
If there's any chance water can trickle down the wires that attach to your electrical box, make sure to use drip loops - basically just a loop of wire hanging down that water will drip off of at the bottom of the loop, instead of travelling all the way into the box.
Some other good electronics sources for basic components - look into Electronics Goldmine, BG Micro, and Sparkfun. Mouser, Newark, and Digikey have fantastic selection; but as you've seen, they can be overwhelming.