Well, it really depends on where you are in the world. Look in the local shops, supermarkets, or some local artisan shops, especially the Italian ones, which most likely will have this. Here in London, UK, we can find it in any supermarket, some brands make it, but I find that the best are the ones made by some Italian artisan shops (I have found a really good one in Hampstead) that have a very good and delicious Nduja paste. The best one I have tasted so far.