If you are not involved in importing or locally producing something for the Chinese domestic market but merely sourcing in China for export, I suggest you start making intermediate- and long-term plans to do just that: move on to another country. By now it's pretty clear which way the winds are blowing on China costing, for nearly all inputs. For many items, it's been an illusion for awhile that China is a low-cost country--when you figure in what's been circumvented in China to keep those costs unsustainably low, it's not such a sad prospect for China to start being on the Losers' End for a change.

As to your current train woes, I just took a peek on availability on the official website and it looks like things are back out to 10 days, but on weekdays, it really is interesting how most trains Ninghai-Shanghai are sold out or nearly so, except for the evening train. Looks like a lot of the seats are being snapped up at Wenzhou, further up the line, leaving a lot less available for the intermediate stops like Ninghai. Maybe it is just legitimate sell-out, but there could be some shenanigans going on also on this route and the fact that you saw availability but were told they couldn't sell the tickets leads me to believe that somebody (company?) has an "informal" arrangement going on with whatever seat allotment Ninghai has. You're not doing anything wrong. Unfortunately, not really any other good and cost-effective alternatives that I can ferret out. What about taking a bus Ninghai-Ningbo (or reverse) and then train on the Ningbo-Shanghai segment? This won't help of course, if the problem is also on the latter segment rather than just on the Wenzhou-Ningbo part.