The Deepseascale in Pokemon Diamond (Which increases Clamperil's Sp. Defense Score and evolves it into a Gorebyss if held during a trade) can be found on wild Chinchou, Lanturn, and Relicanth. Chinchou and it's evolved form Lanturn can be found on Route 220 (South of Sandgem Town) and Relicanth can be found on Route 226 (East of the Survival Area). Either way, you'll have to use a Super Rod to catch them. Use the Thief attack to get one or just just catch one. (Note: They won't always be holding onto one so it could take a few tries...)
You can improve your chances of finding one that is holding it by having a Pokemon that has the ability "Compound Eyes" in the front of your party...