1. They're very rare, but found in a lot of places (I was lucky enough to find two before I beat the game)... good luck.
2. You can trade for one in-game (as mentioned earlier)... if you can tolerate it's nickname (or, get it from the trade, then breed it).
3. After you beat the elite 4, there will be an advertisement on the tv in game (similar to the one for the latias, berry blending master, lilycove blowout sale, etc) that tells you that swarms of nuzleaf and seedot appear in ____ ... where _____ is equal to a random location... This only occours for about an hour after you see the advertisement, so you'd best hurry.