I have been finding and selling discontinued products for 6 years online now. People seem to think there is a magic list, or a site you can go to that lists all these things. Don't know where that idea came from, but that isn't how it has worked for me. Here are a few ways I have found out about these things in the past.
1. PAY ATTENTION. Look at store shelves in regular stores (grocery, Wal-Mart, Target, drug stores), especially health and beauty aisles. Look for empty spaces on the shelves where a product should be but it not. Read the tag and research the product. You may be able to find it somewhere else.
2. Along the same lines, look at the signs stores display that say "temporarily unavailable" or "sorry out of stock." Research those. Sometimes the store doesn't know the situation - they just aren't getting any more of a popular product so they put up the sign.
3. Listen to the people around you in the store. This is the best way to find out! This happens to me all the time. People are shopping, the product they want is not there, they either say something out loud to themselves or to the person they are with, or to a nearby stock clerk. I usually go like this, "Ugh! Why are they always out of ______? I am so sick of this! I can't ever find it anymore!" If they don't say the name of the product in their rant, ask them what it is. They will tell you even more about why the product is so great and their frustration finding it.
4. Listen to the national news, or read trusted sites like USA Today, Huffington Post, New York Times. You will find lots of info on products being taken off the market, a plant being moved, an ingredient being investigated. I am not talking about recalls or product bans, but just news about product productions. Happens all the time. Case in point - the Today Sponge shortage was on the news.
5. Pay attention to your own sales. Is something selling unusually fast, or are people buying in larger than normal quantities? Has a slow mover now become a fast mover? Last year I was selling some hard candy, and I noticed as soon as they hit FBA I sold out, and the buyer would buy all I sent. PAY ATTENTION!
6. Talk to stock clerks and understand the store's markdown and tagging system. Ask questions about what the tags mean, DON'T ASSUME. For example, when Wal-Mart is not going to carry a product anymore, they put a red sticker on the shelf where the price is displayed. The sticker is not put on the product, and it is not always marked down. It is not always put in a clearance section. Just the red sticker on the shelf. The product may not be carried anymore because it isn't profitable, it isn't popular, they are making room for something else to go there, or the manufacturer is discontinuing it, there may be a packaging change and it is coming back later, etc.. Look for the red stickers and investigate on your own further. The stock clerk only knows that it is going away from the Wal-Mart shelf.
I have several right now that are in short supply and I learned about them through the methods above. Keep your eyes and ears open!
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