The belt is an odd-sized T-belt (timing belt), the odds of you finding the exact same size at an auto parts store is very slim.
Just about anyone who sells a 4G should also sell belts for it. This is a slow time tho, many places are out of stock of parts. You might try calling Helio to see if they can sell you a belt for that 100T pulley. Their number is on the Contact page.