Hi Boo2, both of my local Sainsbury's sells it. It's in the specialist food section (where you get all the weird and unusual spices, olive oils, pasta etc.).
Most Sainsburys seem to have this section, but it's normally hidden away on the end of an obscure aisle!
Not knowing your location, but is there a Wing Yips supermarket near you. They are a nationwide outfit.. A quick Google should find the nearest to you.