Don't know how easy it is to find a Rainbow, but you could also look for an Epson QX-10 Z80 (CP/M 2.2) or QX-16 Z80/8088 (CP/M & MS-DOS) if you want...
I recently got my hands on some Zenith Z-Station 433Dn pizzabox machine. It works but CMOS battery was discharged and it lost all the BIOS settings....