Very straightforward achievement and can be done anywhere, however I found an area to be particularly easy and close to a shop. The area you can fast travel to is called the Ghost Cat Mine and is about 150-175km from a river. The Ghost Cat Mine is in the South Western corner of Faith's Region and is just over the border of John's Region (see Screenshots). The river has plenty of rainbow trout which seemed very easy to catch. My bag allowed me to catch 10 at a time, then I'd run back to Ghost Cat Mine and Quick Sell the 10 at the Garage. Run back to the river, grab 10 more, back to the garage to sell em, and the achievement is yours.