Not long ago I was in the difficult position of batching cocktails for 200 people. Apart from the usual mathematical challenges I face, I struggled with an appropriate balance of brown and boozy vs. easily drinkable and refreshing. I settled on Old-Fashioneds and La Palomas—the latter outstrips the margarita in popularity in Mexico. While so much can go wrong with a margarita, La Palomas are easy, featuring just tequila, lime juice, and grapefruit soda.* The last is where you can go wrong. You could use some high-fructose-corn-syrup-spiked swill like Squirt or Fresca, or one of the many smaller Mexican brands that use HFCS. But Jarritos is like Mexican Coke, sweetened with real sugar. Plus it's mildly bitter, like actual grapefruit, and it's superfizzy. It needs no alcohol to be perfectly drinkable on its own.
Finding Jarritos Toronja in Chicago is no problem in your better-stocked supermercados. If they stock Yakult, they better have Jarritos.
* Don't go all pickled paprika and lavender petals by squeezing your own grapefruit juice. Use grapefruit soda, dammit. For my big 24-bottle batch I extrapolated David Wondrich's recipe. Of course you don't want to batch the soda—just the lime and the booze. When serving, pour the batch mixture over ice in a collins glass and top with the soda and a lime wedge. Your guests will drink it like they're dying of thirst.