Howards Storage have some but like most I think the mirrors are small. I also found a really nice one on Marks & Spencer and they have free delivery in September. (it's not really in the style you were wanting sorry) http://www.marksandspencer.com/square-extendable-mirror/p/p22319481?source=affwindow&extid=af_a_Content_79682&comgp=79682&awc=1402_1411636962_c6aa0b8cc845d5ae78080e37730547c8

have a shorter recessed wall cabinet for storage and if necessary have the balance of the space fixed mirror.....you should find something suitable at a fraction of the cost of custom made but strongly suggest that you don't have the vanity hard up against the glass screen (a gap is useful for hanging a hair dryer or hand towel and allows easier access to clean the glass screen) we've been disappointed with custom made vanity and wall cabinets which were more than twice the price and no better quality

Invest in a custom made one from your local glass and glazing specialists - it'll be made to fit the vanity top exactly and the edges of the mirror should also be sealed to prevent corrosion. (which often happens to mirrors from the hardware stores)

You could always add a small table or a plant to balance the space, this could also stop the door from going back too far and damage the mirror.