If you're looking for a simple way to add a battery to your LilyPad project, the LilyPad Coin Cell Battery Holder might do the trick.
It's a simple board which holds a 20mm coin cell battery (such as a CR2032) and gives you 4 connection points (two positive and two negative) for sewing into your project. It beats having a pair of AA batteries hanging from your head for your brainwave-reading Lilypad hat!
Note: The LilyPad Coin Cell Battery Holder is a design conceived by Felicia Williams of Boulder, Colorado. Also, a portion of this sale is given back to Dr. Leah Buechley for continued development and education of e-textiles.