Cabinets are generally available in 3" increments. So, 24", 27, 30", etc.
42" sink base cabinets are available at any number of online resources.
Have you contacted a cabinet shop or cabinet seller in your area?
I generally work with a kitchen cabinet designer who buys from one semi local large cabinet shop. The quality is good and she knows all the tricks, which I sometimes forget!
Your choices are a flatpack shipped cabinet that you assemble (ikea-ish); a cabinet shop or design shop as i described aboved that will deliver a fully assembled and finished cabinet, or a full custom cabinet maker that will build whatever you spec, to order.
Prices increase accordingly.