While they are becoming more widely available (Trader Joe’s Bomba sauce is a good substitute) if you can’t track down Calabrian chili paste then I’d recommend finely chopping fire roasted red peppers and stirring them together with crushed red pepper flakes to form a paste. It won’t have the exact same flavor but it will do in a pinch. If you can’t find Calabrian chili paste at your local store I’d recommend ordering a jar from Williams Sonoma or Amazon.