Harissa is the name of a spicy Tunisian chilli paste. It is an immensely popular and important condiment and is very widely used in Tunisian cuisine. We have never made our own harissa as it is readily available here. For those people living outside of Tunisia who would like to try this delicious spicy Tunisian paste I think you would be able to find it if you went to a Middle Eastern and/or gourmet food shop.
Alternatively, you could try making your own. Here is a recipe I found on the internet on a website called allrecipes.com - I have not tried to make it however I was heartened to note that the person who posted the recipe had a Tunisian name so hopefully the recipe will be the real deal. Here is a link for your information:
Its main ingredients are piri piri (type of chili pepper), serrano peppers and other hot chili peppers and spices and herbs such as garlic paste, coriander, red chili powder, caraway as well as some vegetable or olive oil. It is most closely associated with Tunisia, Libya and Algeria but recently also making inroads into Morocco. Recipes for harissa vary according to the household and region. Variations can include the addition of cumin, red peppers, garlic, coriander, and lemon juice. In Saharan regions, harissa can have a smoky flavor. In Tunisia, harissa is used as an ingredient in meat or fish stews with vegetables and as a flavoring for couscous. It is also used for lablabi, a chickpea soup usually eaten for breakfast. Tunisia is the biggest exporter of pre-made harissa.