There's a couple of places I have seen them, although none of them especially "convenient" in location. They're just places I remember passing whilst out walking or cycling.
One is Thornsett Fields Farm on Briargrove Road - near the junction with Aspenshaw Road. This is the road from the top of Mellor to Thornsett.
There's also two between Mill Brow to Shiloh Road - where Hollywood Road and Chatterton Lane meet - One person/farm has a box at that junction. But go up the lane to Horsepool Farm and they also sell them for 50p a dozen cheaper if I recall