I just Googled them and you can get them for around a fiver (three in a pack) from various places. They store flat and therefore easily, they won't rust or rot(?) and are made from recycled plastic..... coffee cups I think.
They are designed to fit in horizontal fencing without additional support. They can also be screwed in place (two holes in top tab) or hung on a hook or a nail in a wall or fence.
I have vertical lapped fencing so it is no use for me in its primary form. I also don't want fixed pots in fixed places so I lob mine around where I want things using 'S' hooks over the top of the fence.
|waiting for its pot|
|brightens up a bit of empty fence|
|smells lovely where I walk past|