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