making an arm that big is quite a challenge, you should look really carefully for an industrial arm.
if not, I hope you have experience with mechanical projects. Calculate the torque your motor+gearing will need, then the speed you want your arm to move with (less speed= more power).
with these you can select a motor and a gearing.
Where to buy these motors? ... I don't know, I make robots that are maximum 1 square foot.