I like the look and taste of salsify, and have had it in restaurants. However, I cannot find anywhere to buy it. I don't even mind how it comes - tinned, frozen, fresh.
I am fairly sure I have seen it in Waitrose, when I had one close by - not sure if they still stock it, or if it is location dependant. They don't appear to have it via Ocado either. Need to be quick though - it's the back end of the season already.
It sounds obvious, but just in case -- you should ask at your local greengrocer. They will at least learn that there's a demand, and try it for one order.
Specialised greengrocers often have more interesting stuff than supermarkets -- I've seen romanesque a lot at my local one, and never in a supermarket.
You could also ask the restaurant where you tasted it, where they bought it.
You can order it online. I don't personally know of any shops where you cab physically go to buy it though.
Fresh salsify is usually only sold in supermarkets when in season. For Europe, this is mainly the autumn and winter months.