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/

Old console guitar equipment can be found online at a higher price than I'd like, but honestly people mention finding the controllers at Goodwill and other thrift shops for next to nothing.

Clone hero feels just like guitar hero with the bonus of being able to add new community mapped songs to give you an endless library to shred.

Harmonix was swallowed up by Epic but they occasionally release "Fortnite Festival Guitars" which IIRC are compatible with Rock Band 4, and if you connect them to a PC somehow they will also work with games like Clone Hero.

For the legacy equipment, you're going to have to go thrifting and hope you find what you're looking for.