Does anyone know where I can find custom LEGO figures or MOCs of the One Piece Manga/Anime, preferably made using authentic LEGO elements?
I'm assuming you are referring to the One Piece anime series.
There are no official Lego sets for the One Piece anime series. If there were, you'd have seen them in the Lego catalogs.
Searching the web, I've found that there is a company called Diamond Blocks that has created One Piece sets. Strangely, they don't seem to have minifigs of the crew.
You could perhaps customize your own minifigs. You could make some stickers and apply them to your existing minifigs, for example.
Or you could contact Lego company and ask them. One Piece is a pretty popular series, so assuming this Diamond Blocks company doesn't have an exclusive license, they might just be interested in creating it.
There are third party companies like KORUIT that have made "One Piece" minifigures. You can check out the review on the KORUIT "One Piece" minifigure here: https://customizeminifiguresintelligence.wordpress.com/2018/12/14/review-of-koruit-one-piece-minifigures-xp036-041-kt1008-kt1013/