You bought it "as is," or you found it at a garage sale, or it just broke. Whatever the reason, you have a piece of ready-to-assemble furniture with a couple of pieces missing. It's easy to find replacement parts, even those little cam lock fasteners.
Determine what parts are missing. If you have a parts list and assembly instructions, it's easier. But even if you don't, examine the piece of furniture underneath and at every joint. You are looking for missing screws, dowels, drawer handles, and cam lock fasteners or connectors. Obviously if something is missing, all you see is a hole, so look elsewhere on the piece of furniture to identify what is supposed to go in the hole.
Determine the manufacturer, if possible. If your piece is new, it is probably under warranty and you can get the missing parts for free. If not, the manufacturer may sell you the missing parts. The three largest manufacturers of ready-to-assemble furniture are all in North America: Sauder Woodworking Co., Archbold, Ohio Bestar Inc. in Toronto Canada and Bush Industries in Jamestown New York. None of them have retail outlets, but all three display their products on their websites.
Contact the manufacturer for instructions and parts. If you have the carton the furniture came in and it's a Sauder product, the company will send you an instruction booklet just from the numbers on the bar code. Sauder's website also has the most complete help for identifying a model number and part number. It also provides generic instructions for many assembly steps.
Bestar's website lists its product lines and a customer service number. But you'll need to have the instruction manual and a model number to get help.
Bush's website shows its products and where to buy them. But for parts it gives only a form to print and send to the manufacturer. It requires a part number and description and the name of the store where the furniture was purchased.
Go to a hardware store for generic parts. Carefully remove an existing screw, drawer handle or cam lock fastener to take with you to the store. (Don't remove a dowel. Just estimate its diameter). Big box stores like Home Depot and Lowes have wooden dowels, and those hard-to-find cam lock fasteners (or connectors). The company websites may not list these tiny replacement parts, but they're in an aisle waiting for you to find them. Wander through the furniture hardware aisle or ask a sales associate.
Even if you don't know the manufacturer, Sauder and Bestar have some good tips on furniture assembly that could be useful regardless of the brand. Customer service representatives may be able to help you determine if their company's parts will fit your need. Next time, save the instructions after you've finished assembly.