6800 and 9000 are going to be one of the few exceptions to what most every one is accustomed to. With those to groups Shimano has once again pulled one of their proprietary moves and gone to a unique 4 point mounting systems instead of using one of the industry standards. Subsequently, to the best of my knowledge, you can't get rings "yet" from aftermarket sources. I'm sure this will change. Furthermore, while Shimano's new mounting sytem allows the use of everything from 34/50 to 42/54+ on one crank, they (shimano) are only selling rings in matched pairs. Apparently they've designed each set to work together for best results and subsequenlty claim that mixing and matching would result in recuded shifting performance. I'm completely unaware of how much shaping or sculpting of the inner rings they've performed. It seems hard to believe it would be much, except in the case of triples.

Anyhow, for now, if running 6800 or 9000 you're stuck with buying pairs of rings from Shimano in 50-34T/52-36T/52-38T/53-39T/54-42T/55-42T and 46/36T. Mix and match at your own risk.