You can use a cheap ziploc-style plastic bag to store them until you get a proper AA battery case. You can find them here:

• AmazonSmile: Search for AA battery case

I like the glow in the dark ones that can snap together to make small or large battery holders. The ones I bought look like these:

• AmazonSmile: Pack of 10 PCS Clear Luminous Glow in Dark 4 Cell AA/AAA Battery Case/Holder: Gateway
• AmazonSmile: Pack of 9 PCS Clear Luminous Glow in Dark 4 Cell AA/AAA Battery Case/Holder: Gateway