Like any tea, yerba has a long shelf life if stored properly. It's true it used to take some sleuthing and a bit of luck to track down a bag of yerba at a nearby supermarket in the U.S., and even if you managed to find some you were unlikely to have more than a couple brands to choose from (in a supermarket in Buenos Aires, there are typically dozens). But these days you can order just about any brand of yerba you like on Amazon for the same price or less than buying it in the store, and it will be FedExed to your door in only a few days. As I write this, I'm drinking a mate made with a very fresh bag of La Tranquera. Bárbaro!
There's some brands in south Brazil that sell in a vacuum bag, like coffee, that I store for an entire year.
This is how I'm "importing" the good stuff to Rio from south Brazil or Buenos Aires, Argentina sometimes. It's easier to find good brands in Auckland, NZ in specialized stores than here.