That is easily found in any true semi custom cabinet line. They have a whole series of vanity pieces at 18", or can reduce depth to 18" from the kitchen line. You don't find it in RTA or stock lines.

What about 15"D pantry cabs but skip the drawers? (Drawer hardware can take a couple of inches (or more) from the depth and width, depending on the hardware used.)

Or, 12"D?

IKEA makes 15"D cabs.

For 12"D, consider using 30" tall upper cabinets and adding the 4.5" tall typical toe-kick or platform. That would give you the 34.5" tall typical base cabinet height. You can probably do this in a stock line. I don't know if making the toe-kick or a platform will offset the cost of just getting semi-custom cabinets or not, but it's worth looking into.