Read the ingredients, and try to match them up at the drug store. If you have an old fashioned privately owned pharmacy in your area, go there and ask the druggist about it. They usually know a good alternative, or maybe they can order it for you if it's still available. Bring the old container if you can.
A lot of these products that work well get stomped by their competition. Example was Quelidrine, an excellent cough medicine. It had a small amount of ipecac in it (usually used to cause vomiting) that in a small dose really helped clear up phlegm and stuffy chests. I haven't seen it in years. I always used it and recommended it when I worked in the pharmacy.