AO2 is the first campaign where certain items only drop from certain bosses, so for everything before AO2, the answer is any map, any boss. The only exception (besides AO2) is that Zyla in the hidden passage drops some level 19 pinks that don't drop anywhere else, and some fathom pinks drop from the boss in the cemetary and nowhere else. But those are both new things and only apply to those low levels so far, though I'm sure we'll see some new gear when the swamp town, and others, come out.