If you've seen the cabinet you want in a Keystone RV, your dealer can order it for you if it's a current or last year model. If it's older than that or if you haven't found the correct size, then your dealer probably can't get it from Keystone. Keystone only builds specific sizes and only in the sizes they need. They don't build custom cabinets. Your dealer may know of or have a local source that does build custom cabinets, but that will probably be a very expensive option.
If you're looking for a cabinet older than a couple of years, you may find it at Trekwood. Their web address is:
http://www.trekwood.com/ and you can link to any of the Keystone models by using the drop down boxes in the center of the page.