storing extra shoes

I have been all over the internet trying to find information on how to store extra shoes. We have 4 at home, but have 5 grandchildren that are here often, and are relicensing to foster again, so I like having extra shoes on hand. They are shoes that have been picked up at yard sales, thrift stores or outgrown by one, but not quite the size needed by the next. I had them in a rubbermaid container in the garage, but found mold on them recently. Vinigar and water for the church shoes, and machine wash with bleach for tennis hoes. They are all clean, no mold, after several days. But I don't want to risk it happening again. Any ideas? Storing each pair in it's individual box is not an optiopn (cost and space). Thank you for your help.