Spicy salad greens: mizuna, baby and wild arugula, baby chard, radicchio, escarole, spinach, baby kale.
Most grocery stores will carry a similar blend or choose your own greens.
Mexican sour gherkin: Can be substituted to thinly sliced Persian cucumbers, but they will not have the same tart flavor.
Edible Flower Petals: You can find these in the same section as the fresh herbs at Whole Foods
Find it online: https://thismessisours.com/summers-end-salad/