Same way you fix neon tubing that is damaged - you pop off an endcap and then carefully break the internal vacuum, then cut and remove one end of each. Place a new exhaust tube on one tube and then use an oxyhydrogen torch to fuse the 2 tube ends together. Place a drop of mercury in the tube when cool and revacuum, and add the right amount of neon gas before sealing again. Place end caps on and there you have a non standard tube.
Otherwise you have to order plain glass in T12 or T8 size, phosphor and sacrifice a tube for the electrode assemblies to do the same, though you would have to be careful in pouring the phosphor coating into the tubes. Much better to let somebody make them in bulk and just join them.
@SeanB: In such you would have to replace both ends or work in an inert atmosphere, as once cathodes are activated (that happen during tube evacuation), even short exposure to air oxygen damage them forever...
With "neon" sign tubes this is possible even in the air, as these use cold cathode concept, where cathodes are made from regular metals without any sensitive coat. In hot cathode lamps the cathode coat is applied in the liquid form onto the filament, then dried and during evacuation process (when there is no oxygen anymore) heated up, what trigger the final chemical reaction forming the final operating composition. But that composition is then very sensitive to oxygen...
Otherwise you have to order plain glass in T12 or T8 size, phosphor and sacrifice a tube for the electrode assemblies to do the same, though you would have to be careful in pouring the phosphor coating into the tubes. Much better to let somebody make them in bulk and just join them.
With "neon" sign tubes this is possible even in the air, as these use cold cathode concept, where cathodes are made from regular metals without any sensitive coat. In hot cathode lamps the cathode coat is applied in the liquid form onto the filament, then dried and during evacuation process (when there is no oxygen anymore) heated up, what trigger the final chemical reaction forming the final operating composition. But that composition is then very sensitive to oxygen...