You can access Grand Arena Shop by clicking Merchant in the Grand Arena menu:
You can also select the Grand Arena Shop tab when you open any other Shop:
Here you can spend Grand Arena Coins to purchase hero equipment and Soul Stones of Lian, Markus, Ishmael, and Rufus.
New goods appear every 24 hours at 9 PM your local timezone. Also, you can refresh the Shop using emeralds.