Just like Henry, I'm going to recommend that you try to buy from a coop. In this case, it's Titrayju (Tierra Trabajo, y Justicia) that has a fine product in "Yemico". Its not mild, but because it's dried over hot coals (not under a lamp, if I've got my facts straight), it doesn't upset your stomach (cause acidity).
The real reason I would buy the product, though, is because you can help to redistribute wealth in Argentina (you know, what Jesus would be all about). I would never give one more cent to the makers of Rosamonte (and I think they make other brands), and here is why:
In 2001 a kilo of Rosamonte cost 1.75, but by 2005 (I think) they had virtually tripled the price of their product. Can anybody explain why a company —that sells probably 93% of what it produces on the local market— would have to keep its prices stable in dollars? If memory serves, they were the first manufacturer whose price tripled after the 2001 devaluation. And, if it wasn't the first, it was certainly a FULL TWO YEARS AHEAD of most other supermarket prodcuts.
¡Abajo la mentalidad de la oligarquía! I'm sure they are nice people and all, but let's not support such ruthless capitalist practices (unless you agree with them, in which case please continue thinking that the world is a great place to live in). Finally, I think it's been documented that the people who harvest yerba mate are still working under 18th century plantation-like conditions.
To get Yemico delivered to your doorstep, order at least 4 kilos (give some away if four is too much). The number in Bs As is 4958- 0679. If you happen not to like it, please buy another non-corporate brand. I've been drinking it about 3 years and of about 15 Argentines I've recommended it to, 2 didn't like it (one of whom, after several other excuses, wound up telling me it was her teenage son who didn't like it ¡Cualquiera!).