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Aniomagic has a great electric sewing kit that lets you easily create your own conductive fabric switch that powers an LED sequin. The kit contains everything you need, even a battery, and is only $15. There are a bunch of great examples on the site too!

The Electronic Sewing Kitâ„¢ is a set of materials that let you stitch electrical circuits into fabric. Silver-coated threads and tin coated fabrics feel just like traditional thread and fabric, but conduct electricity! These novel materials let you build soft, squishable and washable circuits that can decorate personal artifacts like clothes, caps and bags.

Electronic sewing kit – Link