Yes you should ask. But I think you'll find in 3D communities (including Polycount) half the people can't even be bothered to credit the original artist, much less ask permission. So if you want to be a douche about it most people here probably aren't going to bat an eye.
Pinterest is a great place to search for concept arts. Follow boards of people who have game art/environment/character/reference images/weapon (whatever you want). artstation and polycount's 2D art section also has concept art. and yes you should ask before using any concept art.