Hello and welcome Toby.
Looking at the parts diagram for the mirror, it would seem the mirror case is a complete unit on the 2004 models and doesn't consist of a head with a separate cap.
That's not to say there aren't aftermarket caps, but the only ones I could find were carbon fibre ones which aren't particularly cheap.
Either repair the crack and repaint it, or find another from a scrap car the right colour.