Contact the manufacturer. Even if the furniture line has been discontinued, it is possible that it will be available for purchase either directly from the manufacturer or from furniture outlets or catalogs. This is always a good first step to see if you can find that missing piece to complete your collection.
Try to match stain and style on new pieces with your existing pieces. The most important components that determine whether furniture matches are the color or stain of the furniture and the general style. If your furniture is French country, for example, you should be able to find other French country pieces from different lines that have the same general shapes and styles. You can match stain by bringing a drawer from a dresser with you when you shop, or by finding out exactly what stain was used on your furniture. Cherry wood will always look similar to other cherry wood, for example, especially if you don't have the pieces right next to each other.
Adjust little details. If you can't find a matching dresser for the existing pieces in your bedroom but you find one that is close, you may be able to make it appear to match by switching the drawer pulls to match the drawer pulls on your existing pieces. Hardware like this can be purchased at any hardware store and drawer knobs can easily be unscrewed and switched out with a screwdriver. By making sure these details all match, you will be able to create the illusion that everything is part of one single matched set.