@Felthry
If you've got a Mexican grocer near you, they're likely to have some. Jarritos and Favoritos are brands I've seen.
I've always been a little jealous of how many more soft drink flavors they get that are rare here. Apple! Lime! Even sangria, which is like orange plus red wine in flavor; the soda itself has no alcohol, though.
@Felthry
In my experience, tamarind is a very like-it-or-hate-it thing. Maybe people just here aren't into it enough for it to be more widespread.