HELP! Flow switch for Automatic Ballast

I can't find a decent price on a flow switch to complete my ballast system! Where do I get them and which one do I get? I hear Toyota boats used to use flow switches on their systems, what kind were they???
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(In case you're wondering a flow switch completes/breaks a circuit when water is flowing/not flowing through a hose/pipe.)
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I am running dedicated simer bw85 pumps with a reversing switch to each sack, plumbed in through the bottom of the boat. I want the flow switch in line to tell me when the sack is empty so that I don't burn up the pump sucking the sack beyond dry, since the discharge will be below the waterline and the only other way to check is to open the compartments and watch the sacks.
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The cheapest flow switches I can find are around $100. Surely they can be bought for $20 - $50 Help!

I can tell when my sacks are empty by the sound the simer pumps when they run out of water. The impellers are cheap too, the money you spend on a flow switch can buy 6 or more.