You are going to be looking at spending a large amount of money on buying a rare clown, if you want to buy a mated pair its going to be even more money. The process is time consuming, difficult and a pair can take over a year before they start breeding.
That said, occelaris are probably the "easiest" to get to breed.. some variations like the platinum and black ice sell for a nice chunk of change because they are designer breeds.
This thread is akin to asking what dog breeds are rare so you can start breeding them in your backyard.