Deadrock Prison has a ton of them in the water to the southwest-ish.  Around +1200, +460 and thereabouts over a fairly decent range.  Some are at water level, and many are underwater...if YOU get into a 'feeding frenzy', don't forget to come up for air every now and then!  ^_^ There are more in and around the waters of Whitevale as a whole, but that area down by the prison has an excellent concentration of them.  Whitevale (and the auction house!) seems to be the only real source for them.

There are a ton of Shellarks in the water south of Deadrock Post, in Whitevale.