Clip-on Christmas Tree lights
What you want is Coin Cell batteries. Of any type, with a battery holder.

What you don't want is an off the shelf battery holder. If this is a commercial product you probably want to make it look nice which means a custom injection molding enclosure for the battery and lights. Learn to use a CAD program or hire a mechanical or product engineer.

You also don't want to use coin cells, if you want to market this as a commercial product. You'll get just days of light out of it before the expensive battery needs to be replaced. There is a reason we still use wired Christmas lights everywhere. Batteries are inefficient