I can't help with where to buy one, but can help with looking for the right things to look for when you do find one.
I have a hand made version of these cabinets. If the cabinet is made from wood (which most of them are), realize that wood absorbs litter box smells pretty quickly, regardless of how often you scoop and change out your litter. The one that I own is made of wood, but the inside walls are lined with a heavy plastic liner to prevent the smells from seeping into the wood. It is very easy to open up the cabinet and simply wash down the plastic.
If you can't find anything other than wood, I suggest that you add a few coats of polyurethane to the inside of it, and use the stuff for outdoor use, as it doesn't allow water to penetrate as easily as indoor formulas.
The one cabinet that I had that was all wood was ruined within 6 months. I couldn't get the smell out of it. I learned the hard way.
Hope you find what you are looking for.