Well Tank question..

I live on a farm and we have an Artesian well. In our basement we have a holding tank for the water from the well comes into.. on top of the tank we have a device that when the tank is full it pushes a stopper to the top and then the water quits filling.. this item is a two piece antique and it keeps messing up,, my question to you guys is there currently something on the market I could use that is similar to what I've described.. I don't know exactly what you call this thing other than a float stopper .. any help would be greatly appreciated!

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