The wiki of Guitar Hero simulator Clone Hero has extensive information on controllers [1]. Some of them can still be purchased brand new but overall the most popular option seems to be to acquire (and sometimes modify) a used first-party controller from back in the day, as the quality of newly manufactured guitars can be inconsistent given the lack of demand.

You may also consider trying out other rhythm games such as the home versions of Beatmania IIDX [2], Sound Voltex [3] or Taiko no Tatsujin, for which high-quality arcade accurate controllers are readily available brand new (if somewhat expensive). There are plenty of modern titles that are playable with nothing more than a keyboard and mouse as well, so you are spoiled for choice if you are not looking for a Guitar-Hero-like experience specifically.

[1] https://wiki.clonehero.net/books/guitars-drums-controllers/p...

[2] https://iidx.org/

[3] https://sdvx.org/