If you want a fairly cheap, good quality SLR, have a look at Bronica's SQ-B, SQ-A, SQ-Ai (they all shoot 6x6). If you have more money and want very good lenses, have a look at Hasselblad. As for as rangefinders go, have a lok at the Mamiya 6. It is the only rangefinder that shoots 6x6 as far as I am aware.