What city are you in? I'm trying to see if I can find a food blogger from different places in Mexico; maybe they'll know of a source. Since you're from the UK, you may be used to gelatine sheets, whereas in the US, and possibly Mexico, the powdered gelatin is more widely available. Knox is the major brand in the US. I found a site called MexGrocer (though located in California and shipping Mexican items to the US market) which lists one called D'Gari. It may or may not be right next to the flavored gelatin mixes. It could be with the baking supplies, I suppose.