The 5.6mm Marksman are still available. Hefty premium over 5.5 versions when I last compared them. I am not aware of any other 5.6's although I am curious about Bulldogs.
Pellets that shoot ok in 5.6mm barrels: RWS Superdome, Superfield and Hobby. For me at least.
Pellets that fit well in 5.6mm barrels: H&N FTT.
As an aside: new 5.6mm Wasps are the best pellets so far in my SMK CP2.