I’ve bought One Piece figures from both Japan Figure and Big Bad Toy Store, and both have been reliable for me. BBTS is usually easier if you’re in the U.S. since shipping is simpler, while Japan Figure can be better for Japanese releases or older figures that are harder to find locally.
I’d avoid choosing only by character, though. Try checking what line he already collects—Banpresto and Ichibansho are good for affordable gifts, while Portrait of Pirates is more premium. Also be careful with eBay listings that seem unusually cheap, because One Piece figures have a lot of bootlegs.