The term "store supplies" covers a lot of different things.  You need bags for merchandise, paper for printing receipts, toilet paper and paper towels for the bathrooms, cleaning supplies, and many other items.  

If possible, it's best to buy your supplies in bulk from a distributor who will give you good prices.

When purchasing supplies that will be seen by the customer, such as plastic bags for merchandise, give some thought to the impression the customer will have when they see it.  Do you want to pay extra for bags with your store name and address on them, or a simple "Thank You" message, or is a plain unmarked bag sufficient?  What color should it be?  White?  Yellow?  Red?

Do you want to have business cards printed with your store address, phone number, and hours?

Do you need shopping carts or baskets?  Do you want them to be imprinted with your logo and the name of your store?

Are there signs in the aisles of your store to let the customers know about special deals and where to find different types of merchandise?

Create a unified look throughout the retail store, giving customers a sense of being in a unique place.  This will make them more likely to return to shop again.