When in doubt, Whole Foods will usually have Guayaki loose tea and bags in the tea isle. Look out for the yellow bag. Also if you're in Whole Foods, walk over to the cold drinks aisle and you'll most like find some canned yerba mate to try out.

 

    Sabores doesn't directly serve yerba mate but they have some awesome authentic Argentinian and Uruguayan dishes. I would place a bet that if you ask the people working there where to buy yerba mate, they'll give you a local international grocer. Let us know!