On top of fullmoon sardines you can catch them with HQ merlthor goby's too. However, Silver sharks are a lvl 30+ fish, so you probably won't catch them at level 25.

The reason they are so expensive is because the fullmoon sardine is a fish used in crafting and for the class quest, so it sells good in it's own right, and to waste an HQ fish getting a silver shark will require a lot of gil. Hence the high price.