According to Trolley.co.uk, which is usually reliable, the only supermarket to stock them is Waitrose. However they're shown as out of stock for online ordering, so you might be out of luck in your local Waitrose. The link's here anyway:
https://www.waitrose.com/ecom/products/porter-marron-glaces/645154-229193-229194
(The reviews are all very good, except from the person who complained about how much sugar there is them! What did they expect?)