Dulce de Leche in Madison, WI
This list of ethnic groceries from the UW might help: http://uwinternationals.org/madfood.shtml
Of those listed, I also know Los Gemelos and La Hispana Grocery have great snacks (Los Gemelos in back of the store, and La Hispana next door at La Concha Bakery).
If all else fails, Woodman's has everything. http://www.woodmans-food.com/
Madison is a *very* diverse city - amazingly so for a city its size. While I don't have specific recommendations, I know that there's a substantial Latino population in the city and some pretty amazing Mexican restaurants, so you shouldn't have any problems at all.
Alternately, put a jar in your checked luggage and take it with you.