Do you mean the small (round) air heater vent, or the large (rectangular) water heating vent?
The round vent cover is no longer made by Truma and is ‘unobtainium’, but the rectangular cover is available at most caravan accessory outlets, but only in white – Truma stopped making the grey cover (sadly) ages ago.
As Tom points out, there is a good suggestion for a substitute round cover in Discussion 2 of his ‘Little Exsis Book’.