Glass mirrors are heavy - some caravans used silvered acrylic mirrors which are much lighter but difficult to cut/drill outside of factory conditions.I doubt Swift will be able to provide a new door but you can check with a dealer and otherwise I am afraid a breaker will be the only other route. I would assume that the cupboard will be fitted to various Elite models for that year so you would need to measure it to confirm. Other Sterling ranges will probably be a different colour.
If you want to replace the mirror if you give any glass firm the size they should be able to replace it.
I edited the title to add Mirror.what about a stick on mirror, BTW you forgot to add mirror to your heading.