I've got decals that are over thirty years old and they are still as pristine as they were new. I store them in either a 3-ring binder filled with document, photo, coin and or baseball card vinyl sleeves. The othe method is large flat boxes.
I have them on the shelf along with my spare parts. The one thing about storing decals is keep them away from any source of water...especially your workbench! A spilled drink or water dish can ruin a collection in an instant. Do your searching and sorting elsewhere or when their are no liquids on the bench. Keep only the decals you are using on the bench in a safe dry place until you trim and apply them. This way if you have to stop or have an accident you're not rushing to get the damp decals applied before they are trashed.
parts.