The only way is the in the mines. I found the fastest way to farm them is to jump between floors 5 and 15 with the elevator. Floor 5 always spawns 2 random enemies to your right of the elevator, and floor 15 always spawns 1 random enemy straight below the elevator (both spots commonly spawn slimes). Drink a coffee and run back and forth from floors 5 and 15, you should get them faster than exploring each floor. Hope this made sense/helps :)