Probably the easiest to find at an affordable price AND still in working order would be a Yashica-Mat 124 or 124-G. Many of these were sold in the 1980's, and they used to sell for $124.95 as recently as 1982. Many Yashica-Mat 124's will have been amateur-owned and very lightly used.
The Chinese maker of the Seagull twin lens reflex has made a good number of affordable 120 twin lens reflex cameras, and new ones are still available pretty affordably.
Mamiyaflex C220's are fairly affordable, and the MAJOR advantage the Mamiyaflex models have over Rolleiflex,Rolleicord,and Yashica models is that they have interchangeable lenses and shutters; on all the others, if the lens diaphragm conks out, or the shutter is jammed, you're out of luck and repairs will be needed. The C330 was nicer, but also costs more. Many Mamiyaflex models were used hard by wedding or portrait shooters.