If you're lucky and it's been a nice and hot summer, you may find them in farm shops and rarely in a supermarket in the summer.
I try every.single. year to grow them because they're really impossible to find in the Highlands, (too cold) and I finally managed to get one to fruit (grown in my greenhouse) last summer. It was still green when the frosts came in late Sept. I washed it, dried really well and kept it on my windowsill and turned it every so often to ripen. On the 27th of March this year, it was finally ripe enough to eat. It was amazing.