If you're fairly low level your Supply Stash will get you a ton of free MP potions at levels 20, 30, and 40. (If you buy a Perfect Iron Hammer you get more MP potions at levels 50 and 60 as well, but it's not really worth it now that the level 50 gear is temporary, even when the Hammers are on sale.)

If you're high level, you can farm up some Dragon Quest items, sell them to NPCs, and then use the Dragon Points on the DQ Rewards page to trade 50,000 DQ Points for a Demon/Sage Event Card, which will get you event gold, which you can use to buy MP charms.