This tiny coin cell battery holder is ideal for small portable or wearable projects. It holds two 20mm coin cells (2032 are the most popular size) in series to generate 6V nominal. (If you want only 3V, we suggest sticking a crumpled piece of tin foil in one of the slots.) 20mm coin cells are popular as they are used in keychain lights, so they are easy to get, and fairly low cost. They tend to have about 220mAh capacity so this holder would give you 6V @ 220mAh with two cells.
The holder is made of plastic and snaps open and closed. You can tape it closed if you need more security than just the plastic snap. A nice extra is the on/off switch up top.