Another unmentioned alternative could be a laptop battery or 4 18650 Lipo cells like used in vaporizers. They have capacities upwards of 3ah and output capabilities of 30+ amps.

 

One trick to help out - persistence of vision. Pulse the LEDs at a bit over their rated DC current, but cut the duty cycle to less than half. Power used goes down to about half, yet the eye perceives almost no change in brightness. You will have to investigate the best combination of operating voltage/current and duty cycle for the LEDs and battery you have.

The same is true if you simply run it DC using half the power. The human eye isn't very sensitive to double/half the light power, particularly if there isn't something nearby to compare it to.

 

Every LED I've ever seen operated on less than 3 volts, most less than 2 volts. The "12V" LEDs I have seen were just regular 2 volt ones with an integrated drop resistor. This is practical as the vast majority of 12V systems are powered by an alternator or generator, the battery is mostly there to start an engine, i.e., pissing away power isn't really a big problem.

The way to be frugal with power is NOT to series up some batteries to produce 12V, only to squander it on drop resistors! I would consider getting 2-3 volt LEDs and powering multiple smaller strings with pairs of the biggest batteries you can conceal within the costume, like D cells if they'll fit, or C, AA or AAA cells if you have no room.

The idea of pulsing them is a good one, except you'll need a microprocessor, or maybe 555-style ICs, but those burn power of their own and of course would add complexity to your circuit -- how many LEDs are you trying to light? Using a cheap, low power MCU (like ESP8266) would give you not only PWM (pulse width modulation) capabilities, but also with a tiny bit of programming, you could sequence them, control brightness, etc. But one complication, each MCU only has so many outputs, in this case 8 or 9... so it really depends on specifically what you want to do.

Might be something to consider, when you're running out, time to conserve.

 

Every LED I've ever seen operated on less than 3 volts, most less than 2 volts. The "12V" LEDs I have seen were just regular 2 volt ones with an integrated drop resistor.

Well, not quite. LED forward voltage varies significantly with color, shorter wavelengths tend to take higher voltage. Red might be near 1.3V, while blue is near 3.1V. (White is just blue with a phosphor on top.) And that voltage is temperature and current dependent, so things move around under use. One should think of LEDs as CURRENT controlled devices that happen to drop some voltage, not the other way around. That means paralleled LEDs need to be the same color, reasonably matched, and thermally coupled or they won't share nicely. A proper drive circuit regulates the current, irrespective of the voltage. Having the LEDs in series forces the same current through multiple LEDs. The RGB strips I have seen use 3 LEDs in series, so 12V is the nearest standard voltage that allows for simple, cheap use of resistors as current "regulators". 12V LED strips do waste some power in the dropping resistors, but those resistors set the proper LED operating current given a 12V supply. Reduce the voltage and blue LEDs drop out before the others. Raise the voltage too much and the blue LEDs see too much current while the red ones only get a small increase. For 12V LED strips stick with 12V drive, and use a pulse width drive scheme to cut back on the power a bit AND get the benefit of persistence of vision.

 

A solution could be lipo batteries...but i'm warning you...they can be dangerous if handled incorectly....you will need a protection circuit as they don't like to be dischared past a safe voltage level per cell....and are harder to charge since they need special chargers.On the bright side a lipo pack can pack around 2500mAh at 11V and be quite light.Another thing is to use PWM for brightness adjust and this way save some power by diming the LED's a bit.Also tho ....rgb light strip leds are power hungry...if you could settle for something less bright than do that.12V 5A power consumption is no laughing matter ...that's 60W.