I know what you mean. I saw one like that (see pic) and the guy wants an arm and a leg for it. Rather go with a new GM licensed replacement. Probably going to get it at Fusicks or Parts Place.
If your jug looks like crud but you've got all the mounting hardware, I'd suggest you check out Parts Place. They list the reservoir and all the other parts separately in case you don't need a complete kit.
If you want to check them out, this link will take you directly to the 64-72 Cutlass catalogue. It's a PDF (protected document format) file. So if you clik on the edit button on the web browser and drop down to "find on this page" use the keywords "overflow bottle". Here's the link:
http://www.thepartsplaceinc.com/cata...ideimages=true Their catalogue is coming along nicely. As they update it they install COLOR pictures of the products.