Here are a few battery choices that work well for cosplay or wearable applications.
If your project has more than a couple hundred pixels, you'll want to head over to the NeoPixel Überguide Power Page to delve a little deeper. Powering is easy for small projects, a bit more involved for big ones.
For smaller costume-sized projects, read on!
This battery pack is perfect for most costume projects. It has an on/off switch, which is super handy, and it will power hundreds of pixels for a couple hours (depending on brightness, colors, temperature, phase of the moon and luck). The nicest thing is that if your batteries die, it's easy to find new ones.
Don't use rechargeables! They don't provide quite enough voltage.
If your project is really tiny -- say, you just want to power the Circuit Playground itself and the lights on the face, or you have just one or two pixels, this is a perfect solution. It's got an on/off switch! And it's really tiny and easy to hide. It won't last all night with more than one or two pixels on, but.. sometimes that's all you need! And it's fairly easy to find replacement batteries when you need them.
I love these batteries for their tiny tiny size. They last several hours powering most smaller projects, and they're so thin that they can be worn inside a bra or hat band without leaving a big bulky silhouette. These are more or less the same batteries used to power cell phones.
They come in a good variety of shapes and sizes, from tiny necklace-sized ones to big 2500mAh batteries that will power your LED vest all night long.
A warning: lithium polymer batteries can be dangerous! They can catch on fire if they get punctured and/or wet.
Read that again.
Now, make sure you're not going to fall in the pool or the ocean with this battery strapped in your armpit. If you're good to go, I have one more warning:
These little wires break off ALL THE TIME. They are not attached very well, and have absolutely no defense against flexing. Luckily there's an easy fix: put a rubber band around the battery, trapping the wires inside, so any pulling will be relieved by the rubber band.
Do this the minute you take the battery out of its shipping box! Don't wait.
Charging Lithium Polymer Batteries
Charge these batteries up with one of these Micro Lipo chargers. It plugs into your USB port. It'll charge any of the LiPoly batteries that Adafruit sells, and there's a little bridge-able solder pad on the back which will make it charge larger-sized batteries more quickly.
You can find out much more about this type of battery here: LIon and LiPoly Batteries Guide