Most companies provide assembly instructions for their products on their website. Consider searching for the store or brand website for the specific information, such as Ikea or Container Store.
Many manufacturers and company brands have websites that not only promote the company, but also provide information for customers. If the company sells a product that requires some assembly at home, they usually put the assembly instructions on that product's page, on the customer service portion of the site or in the Frequently Asked Questions section. Another option is to download a copy of the product manual from the company website, as that may include instructions for assembling or setting up the item.