Where is the best place to authenticate and get the value of an old Hollywood photograph?

I have an autographed picture of actress (and William Randolph-Hearst mistress) Marion Davies. It also has the photographer's name (I was able to find out he was a well-know studio photographer for MGM). There's a little note addressed to someone (probably a fan), and then her autograph. I would like to find out how to authenticate it, and also to find out what (if anything) the photo is worth. Is there a good resource for Hollywood memorabilia (besides just looking on e-bay)?