Make it yourself. Read the ingredients of the "Yogur Griego" or Dahi and you will see that they are anything but what you are looking for. Thickened with gelatin and lord knows what else instead of just strained (this is also important if you are looking for the health benefits of Greek Yogurt vs plain). Also, making it yourself is a lot more convenient than trekking across town to El Galpón or wherever else (been there, done that), and a yogurt machine will pay for itself in no time if you have a family. Seriously, once you start doing this, you will see that it's a no-brainer! After being shown the light by some people on this forum a while back when I met them for something else, I have become a homemade yogurt evangelist - indeed, I am lending my machine to someone at the moment to get them started.
If you have an oven light, you may be able to do it that way (people on the internet report success, but I don't have one, so I don't know). Use Larga Vida (UAT) milk to skip the whole heating and cooling step that seems to be where people go wrong. If I were elsewhere, I'd want to use organic milk (which doesn't come in UAT as far as I know), but unfortunately it's cost-prohibitive for me here right now: http://www.elrinconorganico.com.ar/joomla/index.php?page=shop.product_details&flypage=shop.flypage&product_id=223&category_id=26&option=com_virtuemart&Itemid=42 - I'm currently using Sancor Leche Entera from Coto which works just fine and is around 7 pesos/L at the moment.
Tip: buy a large bowl that fits the yogurt maker, as you can make double the quantity each time that way, and don't have to deal with the mess of getting it in and out of the little provided pots when you strain it.