For a gun crew you could probably use figures from several King and Country lines, they have some coastal artillery guys that could be modified to be holding the smaller shell for the pak 40 as well as various guys with mortars and some of the other kneeling figures such as the one with binos. But I dont know how much you want to spend on figures. These guys wouldnt be too bad. I looked in TFs section on K&C and found them. Not sure if anyone else even produces a figure that would work with this gun.
Dave