I don't remember the exact monsters...

For earlier in the game, if you use shields, Adamantoises in Kashuon Keep and other areas drop Diamond Shields.

I remember enemies in Castle Paramekia dropping Sun Swords and Yoichi's Bows (!!).

And I usually get Berserk scrolls from the wizard enemies in the Warship area. And I sell uneccessary spell books for extra cash.