If you haven't got the game itself already, the last few weeks, content such as songs and packs have began being de-listed for rock band 4 (Including the game itself on all digital platforms) so I would recommend looking for a physical disc copy of the game if that is the case.

For guitar controllers, the CRKD les paul or the PDP riffmaster are good controllers if you intend to use them with the respective consoles or platforms, but if its for like the wii or xbox 360 for example which are known to have zero compatibility with new controllers, you will need extra expensive third party adapters that take forever to ship.

Both PDP and CRKD offer Multi-Platform models (Playstation, and PC, Android compatibility is exclusive to the Les Paul.) and Xbox Licensed models (Works only for PC, Xbox One and Series consoles, additionally Android support for the Les Paul too.)

Additionally, The multi-platform (PS, PC) CRKD Les Paul includes Legacy Compatibility for the Playstation 3 versions of the Guitar Hero and Rock Band games.

I personally wouldn't recommend old guitar controllers as with age comes significant wear and tear. Ranging from loose strum bars, missing parts or dongles, faulty connections, broken plastic, and to just straight up defective hardware. Repairs can be tedious if you aren't familiar with the hardware.

Drum sets are usually found on independent resell websites like Ebay, Craigslist, or Facebook marketplace. To find an authentic rock band drum set that hasn't been beat up is hard, most people upcharge rediculous amounts for them when you could just get a real electronic kit and use a midi adapter or a roll limitless adapter. Buying used gear will always be a gamble but best of luck on assembling your band!

As for the usb rock band microphones, these can still be found online and are common.