Do you get the pieces in random events or is there a game you play or somewhere you can go that has a chance of giving them out..

Or do I have to buy all the pieces from user shops :CC?

I think if you get through the krypts, you can get a piece of it, but I'm not completely sure about that. Other than that, I have no idea where to find the pieces.

They were also crystal shop items, and Spring 2007 giveaway items as well. :)

I know pieces of the pie come in random events. SubetaHQ says they don't restock x:

It's rather strange they are so expensive, especially compared to the Frankenpie pieces. I still need all the Frankencluck ones to get the doctor achievement and title. :P