Label Storage Boxes
Letters for Selling
Sponges for Separation
I use Pot Sponges like those Amazon to fill out my storage boxes, so that the cards don't slide around. That is currently the cheapest way I've found for that use case. In grocery stores like EDEKA I found them for 1€ in a pack of 10 which would be cheaper than the ones on Amazon.
I also use the empty boxes of DragonShield Sleeves. But those were used up quite fast.