Dragon quest metal slimes

It depends on which slime you want. If you want a metal slime you can go to the Quarantomb. If you want a Metal Medley, go to the Bad cave. If you want a Liquid Metal Slime, you can find them at the Bowhole.

All at them appear on the 'Slime Plateau' above Angel Falls, as well as Liquid Metal Kings. Those can be found in grottoes, as can Platinum King Jewels.