Mamiya 645 uses lenses designed for that body. I had 80, 55, and 150 for Super and Pro. As I understand, N series are the latest one for that body. Availability pretty much stops at 150mm. I haven't seen anything longer come up all that frequently. I think, KEH has them under Mamiya 645 MANUAL section.
150mm is a popular portrait lens for that model. It's pretty cheap, too.