Arena Shop becomes available at team level 10. You can access it by tapping Merchant in the Arena menu:
You can also select the Arena Shop tab when you open any other Shop:
Here you can spend Arena Coins to purchase hero equipment of blue and purple rarity and Soul Stones of Mushy and Shroom, Folio, Judge, Dark Star, Chabba, and Astrid and Lucas.
New goods appear every 24 hours at 9:00 PM your in-game time. You can also refresh the Shop using emeralds.