the plastic springs in the center of CD cases can break, and when a couple of them are gone, the CD will fall out of place. regular jewel boxes have a removable tray which can be replaced; cardboard CD sleeves have the plastic glued in place. what product is used by music librarians to repair such cases - possibly a rather dense foam cylinder cut to length?
Did a search on the web and found this product which is the kind of product that I mentioned in my comment above:
But the same search also returned this one which is probably exactly what you're looking for:
Now I'm not recommending these specific products and suppliers (I don't know them and I don't even know if they're available in your region) but by searching for these type of products you may find what you're looking for from a merchant that servers your area).