I think they are hard to find. I made one out of white acrylic. I made a wooden form (block of wood with a 3/4"? half round filed to the shape I wanted glued to it) . Then I used a heat gun to soften the acrylic and pressed it over the form . when it cooled, i cut it to fit into the domelamp housing.
I dont know if I explained it well, but it worked great.
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