Look at the FAQ... there are the original set of 17 plants that existed for the last 6 or 8 years and there were Poppies in that set... You could cross-polinate and collect seeds from these, and get different colors of plants.
This year we got a new set with seeds that are called "Peculuar"... among the "Peculuar" seeds you can get a new kind of Poppies that are more spread out than the original ones... these only come in "Plain" seeds and don't produce seeds or let you cross-polinate or anything...
You collect the "Peculuar Seeds" from groups of mobs that are listed, I'm sure, in the FAQ, by now... but you can get the new Poppies from Wisps among other mobs... Wisps drop other seeds too though, not just poppies.