I presume you are talking about onyx chess sets. You will find them in just about every other souvenir and tourist shop in Mexico. Small sets (about one foot square) run about $ 20 , medium sized sets about $10 more. A set of onyx checkers to go with it should be about $5 .
Do you have to fly home at the end of the cruise? The chess sets are VERY heavy and with todays airline weight limits you can soon find yourself paying a lot in overweight to get it home. If you pack it in your carry on, be prepared to have to unwrap the whole thing for security. On an xray, the pieces, especially the pawns, look like bullets.
I brougt 3 sets home once. It seemed like a good idea at the time, until I had to carry it all around and unwrap every single piece for security.
We are driving from Seattle down to Los Anageles for Christmas anyway so the weight is not that big of a deal. Do you know if it's possible to get nice weood boards down there as well? I would really like one of those.