They are lewmar size 0 old standard port lights. About a year or so ago defender here in the us had some stock. Catalina direct used to sell it by the foot. My Google ing shows it looks like a lot of places are now out of stock. If you can’t find the size 0. I’m pretty sure all of the old standards used the same cross section. Which means, in theory, you could buy a larger size, 1, 2, 3 etc gasket and then trim to fit.
The gasket has a very unique way of attaching to the frame. However I think you could source a similar shape o ring square shape,or even a D shape automotive type and affix it to the frame. Likely wouldn’t pass Lloyd’s stamp of approval, but should keep the water out…