I'm not sure if you can get the parts you are looking for new. They are available used, but not new. Check eBayAU, seller name KactusJack. He has a lot of new parts for our cars.
The used part you are looking for is the mirror gasket. It also appears as if your gasket is there. The gap you have circled is on every Capri (at least on all of mine).
--Russ