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Definitely don't use a serrated knife. Use something like this: [link] or this [link]
Similar to the 12" Kershaw Scimitar recommended above, I've cleaned/broken down a lot of salmon and larger trout with the Victorinox Forschner version of the same knives like this or this. Probably don't hold the edge as well as some, but I can get through 4-6 medium/large-ish fish (12-20lbs each) without needing to do any more than run it over a steel.
EDIT: Just noticed you want a flexible blade. These bigger blades do flex, but not like a narrow fillet knife. Definitely takes some getting used to. I can fillet smaller fish with my 10", but it doesn't flex like a narrower/lighter blade.
I am a meat cutter and I use this Victorinox 10 inch knife and a smaller 6 inch victorinox for trimming steaks. I was wanting to know a good beginner sharpening kit I could get thats not to expensive. I am not sure what counts as a cheap price for stones, but I really don't want to spend more than $40-$50.
I also have this Kershaw pocket knife that has quickly chipped. Not sure if its the metal or what, but would like to know if I would have to buy a separate sharpening kit for a pocket knife.
Your photo looks like a restaurant-supply brand. Fibrox is about as close as you'll get in a consumer-facing brand.
Edit: a boning knife may be a better match. The blade in the photo is long but usually boning knives are around 6":
This is what I use. Get the biggest one you have storage room for: the longer it is, the easier it is to get through a cut in one motion (plus the weight of the blade does a lot of work for you). Keep in mind that this style of knife cuts best on the drawstroke, it can take some getting used to. ETA If you don't like that, this is another good option.
You might also want a boning knife for trimming off silverskin.