You can find mate easily in the States, but OJO if you like the peperine o menta flavors you can get here as I do, which are not so easy to find. I have never looked online to see if you can order them. Finding a cheap gourd is not as easy as finding the tea.
Guayaki brand yerba mate is in organic/health specialities stores everywhere BUT it will cost you 10 dollars. The bag is small, and goes fast if you make mate the Argentine way, filling up the gourd.
But I went to the "international" grocery store, or the grocery store in the area of town with the most immigrants in Louisville where I am from, and found a bag twice the size or the standard size you buy in Argentine supermarkets for only 3 dollars!
There is always ordering online if you don't have a similar store near year. I hear there are several online companies in the US, though I don't know which to recommend to you.