We sell full interiors but we don't keep them in stock for immediate sale.
The options to create the interior are varied and to me the interior is the most important part of a case, the actual case part. So in addition to the plumbing/electrical tubes you can also search for tube profiles and companies that keep suitable profiles in stock as the minimum production quantity is often cost prohibitive.
For starting out I really think it's ok to buy cheap cases and build around the interior unit. Later you might want to buy ours as it comes fully ready, proven, tested and with a reputation for great protection and with a lifetime warranty for the interior.
For parts try Springfield leather, the buckle guy, and Ohio travel bag.
Thrift stores were mentioned and they rarely have cue cases but what they do often have is great bags that you can harvest for parts and material.