the 2 main sources of corn in a margarita are going to be the grain part of 'grain alcohol' and the corn syrupy additives in the drink mixes. you can take these allergens out of the equation by using a silver, 100% de agave tequila only and making your own mix. there are several brands of safe tequila. lunazul, 1800, patron, milagro, etc... and there are several options for making your own mix. here's my favorite:
peel and blend mandarin oranges to about 2.5 cups in the blender pitcher
add 2 cups pure lime juice
1/3 cup agave syrup
1/3 cup tequila for every cup of mix
add 1/2 cup ice if desired
blend at level of pulpiness you desire. i love orange juice pulp, so i blend on medium. for less pulpiness, blend higher. for NO pulpiness, do pure orange juice in a carton and pure bottled lime juice. (check labels for citric acid, ascorbic acid or vitamin c, these are all corn) taste and adjust ingredient amounts to personal taste as desired.
pour ring of real salt on wide saucer or small plate. rub wedge of lime or orange around rim of margarita glasses, then turn upside down and rub in salt to encrust salt on rims. pour margaritas in and enjoy! this should make about 4 margaritas.
you can make wonderful mango margaritas by substituting mango instead of orange and lime. whole foods 365 brand frozen mango squares for smoothies are perfect for this. you can use either real salt or sugar on the rims for this sweeter margarita.