It was called a Game port I think, had a tendency to be included on the older soundcards.
You should be able to get a USB adapter from Maplin - but you will have to pay a fair bit for it. Unless the Joystick is a particularly good one, it may be worthwhile just buying a new USB as you may have issues getting games to detect the old one.
Alternatively, if you're running lots of old games, it may be worth you finding an old Soundblaster card with a game port
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