These days it’s getting hard to find joysticks or gamepads that would work on Amiga, Comodore 64/128 and the rest of the compatible computers. Of course, you can find used ones on ebay and like, but from what I’ve seen most are pretty banged up and/or expensive so I don’t consider them a thing to be relied upon.
Sometimes you can stumble upon new ones, so called NOS, New Old Stock, or if some company makes a series for vintage fans, but again, any option is pricey. In my opinion that makes sense for collection, so you can have everything nice and shiny for display. But if you intend to use it cheaper, more replaceable, stuff would be nice.
But luckily there is cheap alternative. Sega Mega Drive 1, 2, 3 (basically the same console) or Sega Genesis as it was called in North America. It uses the same 9 pin DB9 connector as these Commodore computers. They can even work out of the box on them but it’s highly discouraged. Especially in Commodore 64 case where they can kill it. I’ve seen few versions, for starters I ordered cheapest ones I found on ebay. Price is about $6 for two. They even came in boxes, I expected just the plastic bag 🙂 .