The guys I know around here that have commercial mowing services don't sell used ones until they are really "USED" if you know what I mean. We live in a county with a lot of rocks. I am not sure I would want to used one from around here.
Also, any deck over 42 is going to make my work harder not easier, probably not going to find many commercial units that small.
I will call my two buddies that have (competing) lawn services though. They might have some good ideas.