I suspect that building a replica will probably be far easier than finding an original that someone wants to sell. Then again, stranger things than finding an intact original have happened.
The original hatch was a simple fiberglass box with stiffening ribs. It is pretty easy to build a mold out of 1 x_ lumber and light weight plywood (luan door skins). It would be easier to core the hatch instead of constructing the striffening ribs on the original hatch. For a small hatch like the forward hatch of the Columbia 22 simple extruded polystryene foam (pink or blue board) will work reasonably well, but only if you use epoxy resin (which won't melt the foam) rather than the less expensive polyester resin (which will melt the foam). A little care and a long weekend and two or three short weekends and you will have new hatch.