Plastic parts are almost impossible to obtain. Just re-use it for as long as possible. Yours is split so it may be the end of the road. Balls in the cage can be replaced. Replace ALL balls at the same time, top and bottom, using grade 25 of correct diameter.
Headsets are replaceable items. New headsets include cups, cones, balls, cage or cartridge, seals etc.
Removing the "cone" from your fork takes some subtle yet brute force. Replacement HS should be standard items, depending on model of bike.
