If you are looking for a consumable that restores spells and not an item like the Pearl of Power or Ring of Spell Storage, then you're either looking at:
- A bunch of scrolls, but that is class depenedent, and may limit what your PCs can cast to a set list of scrolls.
- A wand/staff that is destroyed when used, we see a few of these one use items in the index of magical items in the DMG, but none of them quite do what your looking for. You could however, riff off of the Perl of Power and make it more powerful but single use, or make it break after its charges are expended.
- Making another item specific to your realm like a Potion of Arcane Recovery or some uncommon/rare form of faerie food.
Now, those last two option I mention might not fit with your requirement:
As a footnote, I am willing to consider divine intervention, but that is akin to me just making up a DM ruling to get the job done. I need this effect for a story element at the end of a campaign, and would like to keep it legitimate in the context of what is going on.
But creating magical items and consumables is completely within the rules and rights of the DM, as stated in the DMG, where it gives advice about rarity and cost, and building such items (DMG p. 130-143). If you don't want it to seem like Deus Ex Machina, then make sure the elements already exist in the world. Perhaps they see enemies using such potions/food before the event against the party. That way, them receiving them at some point is just par for course, and not some crazy DM hat pull.
I like the idea of a certain faerie food, because you could attach to it a negative side effect for non-fey (maybe elves are immune to effects as well, maybe not) like addiction or a slight buzz that gives them disadvantage on skill checks or the like.
Potion is the smallest departure from the other types of magical items we see in the DMG.
If you are looking for a ready-made magical item, that is consumable, restores spell slots, and is found in the index of magical items in the DMG I'm afraid you are out of options. The few items listed are examples, and classic items that are cannon to D&D world; but they are a small subset of the items that might exist in a campaign. As such, they don't have any consumable currently listed that does what you want.