Instead of trying to find a longer dog bone you can make one out of the existing dog bone. Simply cut it in half, insert each end into the appropriately sized steel tubing (brake line works well) and weld them together. If you don't have a welder or know anyone, a welding shop shouldn't charge too much to do the work. Brazing is an option but requires a torch. I've seen shafts soldered together using brass tube but that's a real pain and requires a really big soldering iron or another form of indirect heat.

You would be better off ditching the doc bones for a sliding shaft or cvd's though as they'd be less likely to fall out and get lost. Traxxas Summit axles are pretty tough and can be cut down or lengthened for different applications.