I got an enclosure kit from here with a multiconsole PCB (brain for controller). I did research on what buttons and joystick I wanted and ordered them from focusattack and arcadeshock. I liked that I didn't have to solder everything and they gave me a useful guide for how to set everything up. I linked a build manual in another comment.

But you don't have to go that way! Depending on your patience and engineering skills you could make a controller out of nearly everything. You could get a cheap PCB from a working controller from a game system you like and a joystick and or some buttons and stick them in an Amazon box if you wanted!

Or, you could do what my buddy did and pick up a kool-aid covered broken street fighter 4 stick off the street, clean it out, and put a new board and joystick in it! It's up to youuu