Unless you mean the white thing then its called a mullion end cap, these are specific to the pvc profile so they aren't one size fits all, you need to know what system you have, using my spidey super powers to me that looks very much like eurocell profile
Eurocell themselves won't sell them to you but rather point you in the direction of their approved network of fabricators. I guess one of yours has snapped and is letting draught in? Also pretty sure they just paint the outermost part to suit as well
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