You just need 24V to power your lights. No crazy math. You can get it with 16 AAA batteries if you want, 4 6V lantern batteries, 2 12V batteries, whatever. You can also get a little device to step up the voltage from one voltage to another. Or you can run the LED's at lower voltage- they'll probably light up, just not nearly as bright.
90 lights is coming in at 3.6W, so I would probably go with the 16xAAA batteries- you can hide 4 of the 4-packs you get at radio shack pretty easily inside a costume, and as long as you get good batteries you will get some decent life. Duracells have about 1000mAh, so you'd get (gradually fading) light for about 6 hours. (3.6W/24V=0.15A=150mA, 1000mAh/150mA=6.67 hours)
Does that make sense?
And be sure to keep track of polarity, they won't light up at all if you get the polarity backwards....
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