The word I am trying to discover is how you would say “thing of the taxpayer”. (Like res publica but referencing the taxpayer rather than the public.) So I suppose I am looking for the genitive form of taxpayer, if that is one word. An online dictionary is a little help.
Since it probably isn’t, I’m not sure if I can make a noun string (“tax”, vectigal, as an adjective?), or if I have to say “thing of the payer(s) of tax(es)”, which probably won’t be quite as snappy as res publica.