Ulrich, I think they called them rubber boots but I don't remember. When I ordered, I ordered the joysticks and boots separately. That is the way they work.
Again, the name is APEM.
This is the reference to one of the ones I ordered: SILICONE BOOT FOR 600H SERIES - JOYSTICK plus APEM DPDT Toggle Switch, (On)-Off-(On), Panel Mount
Hope that helps get you pointed to the right terminology RS uses.