Town Shop becomes available once you complete the tutorial. You can find it by clicking Merchant on the main game screen.
There you can buy EXP Potions (their size and potency depend on your team level), Bottled Energy, and random items of green, blue, and purple rarity.
Town Shop updates three times: at 9:00 AM, 2:00 PM, and 9:00 PM your local time zone.