In Metaphor: ReFantazio you can buy MP items from various vendors in order to replenish MP. Here's where to find them:

Available Items: Water Shops typically offer Fruit Cordials, which restore 30 MP to one ally.
Stock: These items are limited in quantity and may not restock during your playthrough.

Available Item: In Komero Village, a vendor sells Magla Pills, each restoring 20 MP to one ally.
Pricing: Magla Pills are priced at 250 reeve each.

Steamed Buns: In the later stages of the game, a vendor located on Dia Franco Street in Altabury Heights allows you to create a steamed bun daily. Choosing the mushroom option yields a Cosy Steamed Bun, restoring 200 MP to one ally. The bracken option provides a Refreshing Steamed Bun, restoring 100 MP to each ally.
Final Dungeon Vendor: A vendor in the game's final dungeon offers an unlimited supply of items that restore 50 MP to one ally.

Limited Availability: MP recovery items are scarce and may not restock. Use them wisely.
Alternative Methods: Using the Mage Archetype's Magic Recovery skill restores MP when enemies are stunned or defeated in overworld combat. This can help conserve MP recovery items.