If you can't find Thai anise basil and Thai lemon basil, you can substitute Italian basil and lemon balm (the latter is available at farmers markets and specialty produce stores). Lemongrass stalks are available at Asian markets and some supermarkets.
Cut peel and white pith from limes with a sharp knife and discard. Cut lime segments free from membranes, then finely chop enough segments to measure 1/4 cup. Mince enough lemongrass from bottom 6 inches of stalks to measure 2 tablespoons total.
Stir together chopped lime, minced lemongrass, lime juice, bell pepper, scallions, chiles, fish sauce (to taste), sugar, and salt until sugar is dissolved. Toss shrimp, tomatoes, and cucumbers with dressing and top with herbs.