hello dbt, here in the states most light assy are sold as one piece. You could try a salvage yard for the unit. Or look for and after market upgrade and change both.
As to just replacing the glass itself, the manufacturers find it's easer to sale the whole unit and make more profit.
Now if it's just a clear cover with no flutes or light shaping lenses then you could try forming lexan or poly carbonate using a form and heat to mold and then trim to fit and then attach.
Good luck Charles
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