When I got stuck with two $450 paperweights, I contacted Mike Reagan to see if he knew of any sources. He confirmed that the castings were changed and he learned the hard way that the Legacy truck frames are not interchangeable with the TMCC ones. I'm not sure how far off they might be, but I was able to locate a couple from a forum member and never looked into it any further.
While mine are currently in good shape thanks to the new parts, I'm not fully convinced that they won't fall apart at some point in the future also, so I'd be interested in this project if it makes its way to completion just to have some spares. I don't like the idea of being stuck with two beautiful but useless locomotives should the trucks fall apart again.