theres a really simple way to make a mister. on one of these forums around here a while ago i saw a thread for how to make a contraption to keep the humidity up in tanks. what it is is a 2 liter bottle (or something to that effect) with water filled up to a certain height. you drill 2 holes in the lid. tubing (like that for an air pump for an aquarium) will fit thru those holes . one tube will run to the air pump and have a stone bubbler (in the fish section of the pet store) on the end in the water. the other tube will be above the water, and extends all the way to the tank. if it sounds confusing heres a diagram:
'.' =|-----air pump---------- -------------tank
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|_cap_|
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stone bubbler
and you just twist the cap on the bottle filled with water. the bubbler drives humid air up into the other tube, which then makes its way to the tank. now if you (1)took the stone bubbler off the end of the one tube, then put it on the end that dangles in the tank, and (2) made that same tube now attached to the bubbler extend beneath water level, once you turn on the air pump it will now drive water up thru the tubing towards the tank, and hopefully thru the stone bubbler, it will spray a near mist from the stone bubbler. since you wont want it running all day, (3) simply add a timer to it and you can set it to mist for a short time. of course, you will have to find the right timer. if trying to feed the mist to several tanks, add a few T-valves (found in the same section as the tubing) and you can split it off to several tanks.
now i have not actually tried this, although i plan to when i have the time to experiment. i would try a digital timer, because i think you would be able to be more precise about the length of the misting, although i do not know the shortest time increment a digital timer would allow. if you try it before i get the chance, be sure to let me know if it works!!!!