Try searching the kit name and number on EBay.
I found some clear parts for a new 1/144th scale B-52 when I wanted to replace a funky cockpit area for a much older kit.
I think some folks give up on these kits and then try to get some of their money back by selling parts.
Your other option might be Shapeways. They often have better quality 3D printed parts available.
https://www.shapeways.com/marketplace/miniatures