Newbie with a 1991 TS6M

I have been appointed guardian of this 91 TS6M for the foreseeable future. It belongs to my nephew who received it as his 16th birthday present. He bought a new boat and gave this one to me. I have driven it and performed maintenance on it it since new.
It has vent windows that open to the front. One has fallen out and is lost. My understanding is the opening vent windows was an option. I would like to replace the missing window. It would be nice to have the original style that opens, but I would be fine with a fixed window.
Can anyone help me?
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