The only thing that prohibits assignments from being on mls is the agreement of purchase and sale itself. Some developers will allow mls, depends on the specific circumstances...
A good place to try and find some would be the sales offices themselves if still open. A lot of times people will call the sales office and them them they want to assign their suite or they will know which agents to contact. I believe "The Condo Store" has a ton for sale as well if I can recall correctly.
Hope that helps.