I don't think I have ever seen a chervil plant for sale (or seeds for that matter),
Chervil is not on sale in supermarkets here either yet often appears in 'high end dining' recipes. I planted some last year and its brilliant. To my delight it is completely hardy and can be picked throughout the year. There aren't many (any other?) soft green herbs which can be picked throughout cold winters.
It has a faint aniseed flavour.