You will not find any cocoa plantation in Antigua and Atitlan as those places are too high. You need to drive to Alta Verapaz. You can contact Ignacio Cac Sacul, President of the Cocoa Producers Association of Alta Verapaz (ony in spanish) email@example.com and Eraimy Ramirez, a consultant of Funcafe, based in Guatemala City he will provide good advice, as well. firstname.lastname@example.org (same, only in spanish). In Antigua you will be able to buy home made cocoa, you can ask for "Don Juan" a former employee of Hotel Aurora in Antigua, he uses to bring artisanal chocolate from his home place (At the Hotel they will manage to find him) Good luck.
Come visit me at Danta Chocolate in Guatemala City. I can also get you in touch with my two cacao suppliers and see if they're willing to give a tour. One of them is quite close to Atitlan, on the coast side of one of the volcanoes. Are you here for Easter week? If so, most places of business are closed except in tourist areas (which will be completely overrun).