I've asked the same, and yes it's odd because you can usually find patys de pollo at any of the granjas and some carnecerias. You have to ask them to grind it, they will think you're a bit silly. I haven't yet had the balls to ask them to grind me some lamb, I think they'd think me crazy. When I asked awhile ago everyone said just to get a chepo meat grinder in Barrio Chino and do it myself, I have to admit I have yet to do so!
Turkey is very easy to find as smoked or baked breast for sandwiches (pavita), but in full form only available once in awhile frozen.
There is the specialty meat place on Lavalle -- I cannot remember the name of it for the life of me right now, if you search the forum (or if you can click on me and search my posts, not sure if you can do that) it should come up -- try simply typing Lavalle or Specialty Meats and hopefully it will appear.