Try going back to the Pyrite Cave on top of the Pyrite Building in Pyrite Town. Reath (the trainer with the Shadow Remoraid) and Ferma (the trainer with the Shadow Mantine) should be in the room where Miror B. was, and if you weren't able to snag Remoraid the first time, then you should be able to have another chance to snag it from Reath.