It can be tough to find a replacement. The last time I needed to do this I looked for a while and ended up buying a used topper off craigslist that had the same rear window/door as my topper for $100. The truck was old with an equally old aluminum cap. Not sure how old your cap is but there is a decent amount of commonality between toppers of the same size/age, try to find a match for sale used. Lots of guys buying a used truck don't want the cover and sell them pretty cheap, find one that has a hole in the roof or a broken side window and the are very cheap/free. You can usually move the window/door over in a couple minutes, then sell/give away the rest after that. I removed the rest of the windows from the one I bought for spares and used the bare shell as a roof for the pull behind mower at my camp.