Is there a script out there that where you can buy magic at a shop and equip it
like in the Final Fantasy I game?

Not really a script, but I think I have a theory on how to achieve a similar result.

Create each spell under the Equipment tab, give it an accessory tag using Yanfly's Ace Equip Engine, add a trait > skills > additional skill > the spell you want. The only issue/s would be equipping of spells, but perhaps you can lock it through a command I'm unaware of. It isn't the prettiest way to do it, but it's a start, if it all works the way I think it does.

Slight edit: In order for the magic to show up, the character needs to be on a class that learns the skill. :/ Unfortunately, that means you'll have to set it to level 99 and restrict the player from actually leveling to it. It's all temporary workarounds until someone scripts it up.