Try searching Home Depot online. They carry cabinets in just about every width, depth, height, and finish. When we were doing the lower-level bathroom at the cabin, I wanted to squeeze in a narrow closet and ordered a 24” kitchen cabinet that fit like a built-in between the shower and outside wall. I posted pics in the photos thread.
(Obviously, you’re not going for rustic, but I’ve found a surprising number of quality home reno options from HD online, especially cabinet sizes in incremental inches.)