A fixed number of Red Iris Flowers spawn when you enter an Explorable Area; they can be picked up by any party member. They are plentiful in Pre-Searing Ascalon and somewhat harder to find elsewhere. Each Iris appears randomly within a smaller dropzone of the area, so the flowers can be easy to find once you identify those dropzones. You can effectively farm Red Iris Flowers in Pre-Searing, by exiting Ashford Abbey and look for the one that appears in Ashford Village (there is always exactly 1), and in Post-Searing by running to Gwen's Garden where there are always exactly 2.