Any unfinished-furniture place should be able to make you one.

Ikea has a line of bookcases called Billy; some come w/ doors. They'll be shallower--probably 12".

"pantry" might be another term, but sometimes pantry cabinets aren't quite tall enough to hold a broom.

here's one that's not white, but it's the right size
http://www.homefocuscatalog.com/Parent.asp?product=263393x

You might also look into Mills Pride cabinets at Home Depot (you now have to order them, but I think you can still get them there), and see if they've got an 18" tall pantry cabinet.
here's their kitchen product catalog--see page 21.
but perhaps the middle shelf is fixed, which might create a problem. Perhaps you could simply not attach it, though the sidewalls of my bathroom cabinet this shape are in two pieces.
You'd need to ask them how it's put together, to see if you could leave the middle shelf out. Their customer service folks were really nice, the times I had to call them.

these guys sell modular cabinets
http://www.grizzly.com/products/H6463