I am looking for a way to disconnect all the power within a 200A, 42-slot, panel board (supporting single phase, 208VAC). I have found a 3-pole relay that could handle 150A and I could connect the control side to a 7 day timer and install it in a separate NEMA 1 box BUT I was looking for something that would install within the panel board itself next to all the breakers. Does such a device exist?

There are CB boxes made with contactors built in, not an add on feature. The ones I have seen and work with switch part of the panel, usually about 1/2 the panel. I suspect that you probably won't find one specifically for that mounting situation, and I doubt that any you find wouldn't fit the existing box. As far as doing it... not sure if that installation would be allowed by NEC code language. Can you install a contactor adjacent to the panel without creating a problem?

I have very basic question. If you find such a device and turn off all the power, what's going to run the timer?

I just have to ask, why would you want to do this?