it's a question of semantics and technical differences.

You can't find 5 speed cassettes because they don't exist (and never did) The cassette system, wherein the freewheel mechanism is in the hub, and the sprockets slide on, came out when bikes were already up to 6 and 7 speeds.

The older freewheel system, has the freewheel mechanism in a separate screw on module along with the sprockets comes up to 8s. 5s freewheels are still around and you'll have no problem finding them if you search the right term. buy 5 speed freewheel