There are countless avenues these days—from big box stores to small online operations. Just Google "kitchen cabinet boxes" and you'll be bombarded with options. The prices will vary depending on the quality of the materials and build—from particleboard and RTA (ready-to-assemble), to high quality wood with dovetail joints and ready-to-install. I've personally never gone this route, although I would look into this first if we weren't able to build our own boxes.