IMHO you should work on getting good shaving edges from your 8k before you move to higher grit stones.
If your edge is not good off 8k it will not improve by adding a finer stone.

That being said the options for high grit stones are multiple, and depending on price too.
Synthetics : Naniwa 12k SS, 16k GS.
Naturals:Fine Coticule, Thuringian, Escher, Jnat,