hello,
i try to find mother board (working well :-)) from Be Box Dual (PowerPC)
or complete Be Box Dual working fine.
francois
You would probably have the most luck with a full system. Be Inc sold complete systems, and the only time I rmember them having potentially incomplete computers were: (1) the hardware upgrade program, where the older 66Mhz systems had a smaller boot rom flash, so there was an upgrade path to get a new motherboard, (2) the pre release systems often were bare bones. The pre release systems are generally pre-PR1 compatible, so no modern version of BeOS would run on them, and the motherboard upgrades were produced specifically for people to upgrade, so I guess not in great quantities.
My own extra point - there’s no real point in getting a 66Mhz system, they are really slow and you will struggle to run R5.03 on it at a reasonable speed.
Good luck!! 
Your other option would be to modify a PowerPC Mac board to fit the case and to find a way to interface with the IO and front panel. A Mac board would at least be PowerPC. You can find dual processor PowerPC macs that run BeOS also.
You didn’t get the other one to work? Maybe I could have a look at it, and since I do have a working one here I could use it as a reference when debugging…