I think they drop from mobs on the sewers, so there's no real farming spot. Thb, I think you're better of farming empowered stones than mana reformer. Everytime I checked, they selling the same price as empowered stones and since buying one means you have to get an epic stone + pay the fee to craft, I don't think it's worth it to farm them. The buyers are probably few too, since so many people are putting big stacks of emp on the market and undercutting each other..