We've apparently located one thanks to an Argentine friend who works at a restaurant in San Telmo. I think it's something like those connections (or others) led him to someone who is getting one to him straight from a nearby granja (farm). I will try to find out more details, but I would say your best bet would be to ask all Argentines or other locals you know!
Turkey is popular for Xmas here, so you should be able to find the bird in different places such as supermarkets ( frozen ), poultry stores like Avicar, Mercado Juramento ( fresh ), etc. It is not hard to find at all. And if you do not want to roast it yourself, you can always go to your closest bakery store and ask and usually for a few pesos they will do it for you ( people usually take there pork, turkeys and other big items to get them roasted for Xmas )