No mirlitons, ya know! LOL. The hard part, really, is finding the pomegranates to get the seeds. They're mostly only available in late fall and through winter.
I only make them when I have pomegranates. I tried freezing some seeds one time and it just didn't work. There is a Spanish treat called "membrillo," which is quince paste. It's sort of like a fruit leather. It's red and can be cut into small pieces--I make it myself, but I've used it once as a substitute for the pom seeds - but it's not really "right" because it's not juicy.
Other notes - a "pitaya" is a "tuna," which is the pear/fruit of a prickly pear cactus. I find them year-round now at the regular grocery, but it's TX. The "acitron" is not citron but it's a cactus candy made in Mexico. You can use anything like candied pineapple or any of the fruitcake crystallized fruits. It doesn't need much.
There are recipes online. I don't use one anymore, or I'd post one or a link to one. Try ordering them sometime at an authentic Mexican restaurant--sometimes they will make them for you even if not on the menu. From Fiestas Patrias - Sept 15th, through about now are the best times of year. Mexicans are very proud of making them for gringos.