Any kitchen cabinet line can order you one. But they are pretty rare. As are the sinks that will fit in such a small one. Most people start moving to a pedestal sink when the space is so restrictive as to need that small of a vanity.

I found 18" vanities online when I was looking for that size for my remodel. But don't know of the quality. I think they included the counters. I ended up going custom when I decided to make my own soapstone sink and counters. I had an open shelved vanity/sink stand made for me. It cost $785 built, painted, and installed. I provided the sink and the counter.

Enduring's vanity is beautiful, and I think I will probably have to go custom too. The bath layout I have right now allows for a 20 inch vanity so I feel your pain.

I would go with a pedestal, but I want the storage. I do wonder where people store trash cans, toilet bowl cleaner, and plungers without a vanity. We will not have a linen closet either.