Motor and Crank - Sprocket Sizing

I'm building a freewheel crank.

Where are the cheapest places to buy the sprockets, I am looking for 50T on up.

Also, how does one calculate, or know, what kind of speed and torque I would have when changing the motor sprocket and the crank sprocket.

I have 2 motors, one is grossly overpowered, 3000W nominal motor, the other motor is a 1500W nominal motor.

I am building a mid-motor ebike, for hill and stair climbing.

I have checked out SBP, the norm is 48T, which my crank already has, 48/32/22, so I was looking for a hill climbing beast of a sprocket, 50T+, I have seen some all the way up to 70T. I think 50T or 52T or 54T is sufficient.

I will buy those quick on/quick off sprockets used on go-carts.