I will say I looked into have Alex Ross do some work. Even if you could get him to do a sketch cover( which he does) , I would say you are probably looking into $5k + . Which for that money you would be better off looking at the stuff he already has done and just picking one from his art site. Sketches stating around $2k and up from there . Full paintings are starting around $10k.
https://www.alexrossart.com/Sketch_c_31.html
If you are serious about getting a special one done just for you and those prices didn't immediately floor you then your best bet would be as others have said try contacting through social media. Twitter, Facebook, Instagram, and the contact through the Art website contact. Although for CGC SS you would either have to get him to agree to creator agreement form and direct submission or a facilitator. That said this would also be a very long process. Especially submitting to CGC .
This all said another option is to have an artist you like their style but not as famous do the sketch through a facilitator and then have it signed by the specific creators, artists, actors, etc.