Here are some options, depending on the situation:
General:
* Shoppers - This is the most straightforward and common term.
* Customers - Implies they are there to buy something.
* Patron - Similar to customers, but can be more formal.
More descriptive:
* A throng of shoppers (suggests a large, busy crowd)
* A line of customers (emphasizes waiting in a queue)
* A gaggle of shoppers (implies a group of people, possibly friends or families)
Informal/Humorous:
* A herd of shoppers (used playfully, emphasizing a large, moving group)
* A basketful of shoppers (a humorous take on the typical supermarket shopping experience)
Ultimately, the best term will depend on the context and the tone you're trying to convey.