Wag Aero sells plans and materials for their Sport Trainer which is pretty close to a J-3 Cub. They also sell prewelded fuselage cages for those who don't wish to do all the extra work. There are plans available for the original PA-18 Super Cub if you want something bigger than a J-3.

You will want to make some basic jigs for the major components to keep everything straight. I suggest you search the archives at www.supercub.org for photos and old posts about building your own Cub.

You can normalize your welds by heating them with a torch and allowing them to slowly cool in a draft-free environment. Not everyone agrees on how effective this is.

Wag Aero and Aircraft Spruce have all the material you need. You may be able to find a local supplier that will save you some shipping money, but you'll have to do some research on that yourself.

The good thing about tackling a J-3 or PA-18 look-alike is that there is a wealth of information and support available.

Best of luck on your project.