Doug, don't think clips are separate item. Once they break off, you have to either replace the panel (big $$$)or become an inventive genius and find a way to repair the broken pieces. If you have all of the schrapnel from the broken clips you might try buttering up the retainer posts real well with JB weld and reattaching the ends of the clips. Let it dry and add some more between the fillets to reinforce your repair. I got away with this on the passenger door of my '89 300E after the panel was pulled away from the door by an unknown assailant. Thus far it has worked. This design for retaining a trim panel on door sheetmetal is not the greatest. If you're not careful you'll break them every time. MBZ should have some way of repairing or replacing these plastic hooks.
1987 BMW 325