Last August I brought back 3 extra large bags (sealed) into the US and even a couple of boxes of alfajores and they never questioned any of it. They seemed more concerned if I came in contact with any livestock while in Argentina.
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.
Some Middle Eastern people drink mate (not sure which nationalities exactly, but I know Lebanese do, because I worked with a Lebanese man who likes mate), so it may make more sense to check out some Middle Eastern shops if you live in a large urban center.
Some health food stores carry it too, but it'll be the more expensive kinds like Guayaki.