I had the same question a week or so ago. Redcat emailed me back and said they should have some back in Stock in around 3 weeks. I know that the Nitro RC Shop had some in stock and they are who I ordered mine from plus an extra!You can mod V3 to fit in the V1 and V2 chassis. The only difference between Part 50007 and 50007N is the bottom of the GearBox. V3 has 3 round nubs on the bottom of the GearBox to fit the new chassis. As you know V1 and V2 has the square type bottom. Everything else swaps over. Rob suggested also doing this mod for more strength (link below) regardless if you have to modify a gearbox or not.
http://www.redcatrampageforum.com/showthread.php?t=182
If you can't find some let me know and I can send you the spare I have.