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.
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.