Ahhhh, I remember going through this exact same little fiasco.
Of course, each of those materials you need only comes from a grand total of one enemy (each), which happen to be some of the rarest enemies in the game at that point. Dancers can
only be found in the Tower in Twilight Town, in the Wayward Staircase. I can't remember which part of the staircase, but since there are only three areas, it shouldn't take too long to figure out.

Unfortunately, there are only like five Dancers.
As for Assassins, they only appear in (one part of) The Castle That Never Was— I can't recall the exact names of the places, but the balcony that overlooks the entryway, right before the room that holds all of the Organization 'graves'— four Assassins will spawn there, along with Snipers. Be careful to make sure you go after the Assassins first, despite the fact that the Snipers get annoying— I found that while I tried to take on the Snipers, the Assassins would go and blow themselves up, since I wasn't paying attention to them.
At any rate, I'd have all three Lucky-Lucky abilities and weapons equipped, as well as the ring— 'cause with how few these enemies are (meaning you have to leave the world and re-enter to get them to spawn again) and how low the drop rates are, it's going to take for
ever otherwise.
Hope that helped. ^_^