The well known Rapala Filleting Knives from Finland are extremely sharp and most of them come with a sharpener.

Ez Glide 7″ Filleting Knife Razor Sharp Handground Stainless Steel.
Deluxe Falcon Fillet with built-in fine grade ceramic-sticks sharpener.
Soft Grip Fillet Knives 10cm and 19 cm include a single-stage sharpener.
Fish ‘n’ Fillet Knives 10cm and 19 cm include a single-stage sharpener.
Presentation Fillet Knives complete with black tooled oxhide sheath.
Rapala Finn Thinn Fillet Knife includes a single-stage sharpener.

Rapala Products are available from Fly Fishing Tackle Co UK