I had a hard time finding any OE used covers since they become brittle and crack with age. Round or square? New or used? I ended up buying repro's for my 67.
I actually found the squarish 68 cover from gmpd not too many yrs ago. Get the part # and search there and rockauto.
I purchased some from Auto Obsession for less than $4 per and they fit great. Definitely not original but a good replacement.
Maybe try this one?? http://www.ebay.com/itm/69-70-71-72...96&_trksid=p4506.m7&item=400350957509&vxp=mtr
http://www.thepartsplaceinc.com/oca...arList=1966&Search_Keyword=&CatID=1&curpage=3 These might work. Sorry, doesn't link direct anymore. Click you model and year then in the parts number slot just type in "dome lens". They've got plain and ones with the tri-shield.