Availability is a regional thing. Cubs are not as common in my area as the Massey-Harris Pony is. However, John is correct. though, not as many Pony's were built in comparison to the Cub. They are more common here because most of them were built in Canada.
I only know of say 6 Cubs within 20 kms of my house, and I have seen at one time or another at least twice maybe 3 times that many Pony's. Parts are readily available for them as well, although, it will probably be 180 degrees from what I am used to.
I have to get a lot of my Cub parts stateside. Pony parts are pretty common here and David over at Chown Tractor Parts
can usually get hold of a couple of Pony's every year and does have Pony parts in stock most of the time.
Implements are not all that difficult to find. I have a couple sitting in my back yard for my buddy Roger that has a Pony. Sickle mowers, roll-over plows, spring and disc harrows are usually available. I even have a Marvel Schleiber carb for a Pony currently on Granny, which will be replaced shortly I hope.
I even have 2 Massey-Harris potato diggers (chain and sprocket ground driven type) that were pulled by Pony's on a regular basis here. Ellie pulls it quite nicely and it works very well. Lots of Pony stuff on the Island too!
Still, I prefer my Cubs over the Pony. Mostly a personal thing - I like the look, sound and the feel of the Cub better than the Pony. However, both Cubs and Pony's have had a historical place in my wife's family, both were the most common next to the Ford N series tractors.
There are lots of pull behind mowers that will work with the Pony, self powered and ground driven will both work.
There is also a pretty good forum for Pony's over at Massey-Harris Forum
and YT Mag's M-H Forum
, although I have not had great responses from the members on questions I had posed on those boards. I am used to much better.
Hope this helps...