bjv5,
You can try your local hobby shop (LHS), for the Hobbico MicroSizer Chassis replacement that is.
The MicroSizer, if you didn't already know, is the US version of the Bit...And throughout my searches of all the online stores that carry Bits, I've yet to see a site that sold just Chassis replacements...the Hobbico MicroSizer is a different story though, I had gotten a catalog from Tower Hobbies and they will be stocking the 29MHz chassis and 49MHz chassis for like $16.99
I think TinyRC will be stocking the MicroSizers soon too, so Tiny might stock the chassis as well...We'll see!