A factory magazine tube will be tough to find. However, Brownell's sells inner magazine tube kits and any gunsmith or do-it-yourselfer can cut one to fit. Use a caliper to measure the inside diameter of the buttstock tube and order accordingly. I see that Numrich has all of the inner magazine tube parts except the inner tube itself. This will be an easy fix for you.
You need the inner magazine tube, Brownells part number 084-027-001, $19.99. The Nylon 66 has a unique follower and end cap, so I would order those two parts from Numrich. The inner tube will have to be cut to fit and you need to drill a small hole for the end cap cross pin. Good luck!
One other note with the Nylon 66 don't waste your time trying to put a scope on it and keep it zeroed. They shoot very accurately with iron sights but the barrel is pressed fitted into the sheet metal action part and the scope is mounted on this sheet metal action part. The barrel moves back and forth and up and down with each shot a little and the barrel will not be pointing where the scope is. I used a Nylon 66 for years with open sights and did a lot of trick shooting with it. I could shoot things as small as a golf ball out of the air as long as it was going away from me most every time.
