There are a few adverbs for governmental levels, but stately is not one of them.
For example, one quote in definition 2 for the entry "federally, adv." in the Oxford English Dictionary features municipally:
1940 Amer. Guardian 5 Apr. 4/3 The privateers never..reduce their rate unless forced to do so by public competition in the shape of municipally or federally owned [electric power] plants.
Municipally suggests a form of local control, like by a town or county government. In many countries, the next step up would be provincially or regionally, as this example from "provincially, adv." suggests:
1992 J. M. Bumsted Peoples of Canada vi. 190 During the Depression, Canadians thought regionally or provincially rather than nationally.
I've been able to uncover no adverb for state that is the equivalent of provincially or municipally. Instead, I've only found articles that refer to the US levels of government - state, federal, and municipal - as adjectives, while slipping in "federally, provincially, and municipally" when referring to Canadian tax collection ("Policing in Canada v. Policing in the US").
So you would be stuck saying something was state taxable (example), taxable by state (example), or taxable at the state level.