Deep Slugs can be found all over the dungeons - Upper Erebus and The Underswamp typically have at least one in the first main room.
Drop a Nether Cap in front of one, wait a few seconds, and then crouch in the smoke.
I found a Nether Cap inside of an urn and have also seen them in the shops. Just keep an eye out as you go. Multiple sources online say to use the Eyeless Nether Fish, but I could never get a slug to eat one.
Unlike using a Ripper Pome, a Nether Cap will also unlock the in-game feat for changing a slug's trail.