There was an entire thread on this subject at some point.
Couple options:
You can get some American Angler Pro Saltwater Blades off Amazon they will work in the Rapala.
I bought a Bubba Blade kit that comes with two batteries, 4 sets of knifes, charging cord and carrying case.
I wanted to see what all the hype was about.
I own both and I do love my Rapala. No complaints.
The winner for me once you use it is the Bubba Blade Electric Knife. I like the 9" E-Flex in the kit.
It has been great.
Like anything fishing it's personal.
I like them both.
I seem to grab the Bubba Blade first when I am going to clean fish.
Good Luck with whatever you decide to go with.
Here is the Bubba Blade kit that I bought:
https://smile.amazon.com/gp/aw/d/B07KSPR7ZC?psc=1&ref=ppx_pop_mob_b_asin_titleThese are the Saltwater (Shark Blades) I use with my Rapala.
https://smile.amazon.com/gp/aw/d/B001F0KB3K?psc=1&ref=ppx_pop_mob_b_asin_title&th=1