Nowhere, i.e. if you are in the falling elevator the tennis balls will fall downwards if it's uniform but any real gravitational field (apart from something of infinite radius) will make them converge gradually towards eachother as they fall towards the CoG

Nowhere in the real world, but that’s okay since we are dealing with thought experiments. An infinite plane of mass is roughly what you’d need so that the contributions of each element add up to a uniform field.

Nowhere. Gravity is just inherently radial. (But if you have a very large spread it can look fairly uniform, of course. Especially if your measure of “flat” is actually curved to the same radius.)

In the pasture where I keep my spherical cows.