For which kit, the Enterprise or Doyusha Shinano?
There's not much in the way of aftermarket for either kit... you can get generic 1/400 rails from Tom's Modelworks, and I used 1/250 PE rails from Wilhelmshaven card models on my Shinano.
For the Shinano aircraft, you can get away with using 1/285 white metal wargaming aircraft. Most are oversized anyway, so it's not that bad, and far less work than scratching and casting copies.
Regards,
Jeff Herne
Modelwarships.com