This thread here shows a lot of cases. There are some nice displays as well in the thread.
Case-wise, I use an aluminum case I got at a hardware store. The foam that came in the case has slits every 1/2" so it’s easy to tear out pieces you don’t want. I then added foam to the lid. I have 30 yoyo slots in the case with the foam in the lid. (Edit: my case is in the thread I mentioned here)
YYE also has contest bags in various sizes (8, 12, 24) so since you’re looking for 12+ size the two bigger sizes might be appealing to you. I haven’t tried any yoyo specific cases so I couldn’t tell you how they hold up. I’m more than happy with my hardware store case.