I use Pelican 1170

https://www.amazon.com/Pelican-1170-Case-Foam-Black/dp/B0038VETHS

Stores 20 safely, basically mil spec grade protection which is great if you have a lot of exotics or rare pieces. You can buy multiple and they stack too.

For about ten bucks more you can have a pelican case that will hole 6-8 yoyos and comes with the foam and everything.

http://i.imgur.com/oTUSF1B.jpg

Pelican 1170 Carrying Case for Multi-Purpose - Black https://www.amazon.com/dp/B0038VETHS/ref=cm_sw_r_cp_awd_7j2jxb8XAQFSW