You can get a moonfish anywhere you can fish.
You catch them like you catch a normak fish.
Sometimes you wont get a moonfish straight away, so it's all about luck.
They're much much more common in my experience at Fisher's Creek. The first time I found that I spent about 40 mins fishing. I think I wound up with about 50 moonfish and one of them (which I entered in the fishing competition) weighed about 845 grams! I think I had of my 50 moon fish maybe 15 of them were over 150. That rod of champions really helps you reel in the big ones.