When tracking down concept art, you want to make sure you're finding the original, authentic artwork. Try finding the artwork on the artist's website/portfolio, the devblog or publisher's website, or the official art book for the game.
Try browsing the developer's website for old concept art, and concept art for future updates
Use a more informal catalog of artwork, like Creative Uncut, if you're just browsing around and don't need an official source
If you're googling around, try phrases like developer concept art to avoid fan concept art ideas, and include -pinterest as a search term (with the minus sign) to remove Pinterest results from your search result list