I took apart one of my old controllers so many times the plastic sheet that goes over the board wore out. The down button on the D-pad stopped working and when I pressed up it would affect other functions too. I bought a new one after the analog sticks got squeaky from playing too much Socom. That probably wouldn't affect your analog sticks but I would make sure the contacts are clean anyway. As for where you can find parts I have not a clue. Maybe you can find a broken controller online that has what you need.
"Drinking strength from immortal fire"*