Highwaymen Shop becomes available at team level 18. You can find it by tapping the Shop button in the upper-left corner of the Highwaymen screen:
You can also select the Highwaymen Shop tab when you open any other Shop:
Here you can spend Highwaymen Coins to purchase hero equipment and Soul Stones of Faceless, Cornelius, Satori, and Lilith.
New goods appear every 24 hours at 9:00 PM your in-game time. You can also refresh the Shop using emeralds.