I have looked everywhere. Does anyone know where to find this piece, if it is available?

I am missing mine on the passenger side and window is rattling pretty bad.

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if that is the rubber piece that attaches to the door you may be able to get them from 914rubber.com .

the early windows do have more rattle than the later windows especially when they are 1/2 way up/down.

If its the door end cap piece it is available from mikey here for $32
http://shop.914rubber.com/product.sc?produ...mp;categoryId=5

Or I have a good used set available for $40 shipped in the US if you or anyone else needs OEM.

Early part numbers are

914.542.111.10
914.542.112.10

This is a metal piece that is glued to the bottom corner of the window.

You are correct. It does not have a part number. I will see If I can locate and early window/glass and get one pulled for you tomorrow.
-Bruce.