Just reposting something I shared a couple of weeks ago (be advised a couple may no longer be available at MSRP):

I've gone overboard with my USB controllers, from RetroFlag for SNES, Hyperkin for Genesis, Hori for TurboGrafx, the PS Classic controller for PSX, Hyperkin for Atari, and a RetroUSB NES adapter for my original controller (there aren't many good 2 button NES style controller) ... and a crap ton more, ha.

There are fan made retropie images available. I believe those aren't frowned upon (as much?) as they are given away for free. Just search for "retropie raspberry pi 4 images", download an image, write it to an SD card, pop it into a pi and off you go. Quickest way to get started with retro gaming.

As for controllers, the Retroflag wired USB snes controller is an inexpensive and good quality controller to start with.