Now is the right time to pick sloes, they are best after the first frost. However, I'm in the midlands and there are no sloes to be found this year. Even the hedges around our garden, which are usually loaded, are barren this year.
Got some in England last week (enough for the sloe gin) but still few enough on the bushes there too. Maybe to do with the weather this year.
I've never seen sloes sold in a greengrocer / shop.