After reading the linked article with a tequila / lime juice / grapefruit soda cocktail recipe, I'm curious to try it. They recommend Mexican Squirt or Jarritos. I'm not sure where around Boston to find these or any other grapefruit soda, not usually being much of a soda drinker.
My best guess so far is the Brazilian supermarket on Somerville Ave., but I haven't been. Any thoughts? Cambridge/Somerville or T-accessible preferred.