I still have the original boxes for the majority of my Game Boy collection, but I don't actually use them for storage.
I keep all of my GB/GBC games in
Nyko Deluxe Care Cases:
The cases are made of hard plastic, have a built-in clasp on the top, are available in multiple different colors (at least 6), and each case holds up to 12 games across 4 rows that flip forward/back for easy access.
I think I have at least 16 full cases at this point but I could always use a few more, haha! It looks like a few 3rd-party sellers are selling them for ~$10-12 on
Amazon if anyone else is looking for one.
As for GBA games, I used
custom cases made from DS game cases for a while:
I ordered the cases in bulk from Nintendo, trimmed the inner plastic to make room for the manuals, and got the custom inserts from
The Cover Project (I actually created/uploaded the Zelda insert and a few others back in the day).
It became prohibitively expensive to keep making a custom box for
every GBA game I picked up, so now I keep all of my GBA games in
trading card sheets in giant 3-ring binders:
I currently have 2 full binders - each binder fits 10 sheets and each sheet holds 9 games.