Float Switch and Valves
The float switch is a ball or cylinder or pear shape that is at the end of a cord. The range is adjusted by how close you attach it to something like a vertical pipe in the sump. One that I use is Stock No 29226 at USABlueBook for $27.94. It will handle 5 amps and has SPDT contacts. You adjust them by where you attach them to the pipe and you adjust the range by the length of cord between attachment and float.
A common brand of solenoid valve is Asco, sold by Grainger and others. Grainger also has their own Dayton brand. They are going to run $50 to $80 each depending on size and brand. You will want one rated for water (brass or stainless or plastic) and for a pressure that is greater than your supply pressure. If cost is a consideration you might find an appliance valve as for a dishwasher or washing machine, or shop on E Bay if that is your style.
You can get the kind used for watering lawns for a lot less but they are not as reliable and can't be used on hot water. They are also 24 Volts and will require a transformer and are 1" pipe size. They are available at Home Depot and other places. I don't know about code issues related to putting them in a house.
One problem with solenoid valves is that there aren't many that are "Normally open" and the "Normally closed" ones will consume a significant amount of electricity if they have to be on all the time.
To select the valve size you will need to know the flow you need and the pressure available. They usually specify a value of Cv. Multiply the Cv value by the square root of the pressure drop that you can permit across the valve to get the flow in gallons per minute. For example, a valve with a Cv of 0.34 with a pressure drop of 25 psi will have a flow rate of 0.34x5=1.7 gallons per minute. That is plenty for a toilet but maybe not for a shower. Ignore the number in the tables that says max flow. That is just Cv times the square root of the maximum rated pressure and has no meaning because you won't operate at that pressure.
An alternative to the cost and effort of solenoid valves is to hook the float switch to an alarm. You can get an alarm that would certainly get their attention and could not be ignored. Grainger has couple (Stk Nos 4NE49 and 4NY92) that run off a battery and include the sensor. They also have float switches if you want to get everything at one place. Anyone who has a business can buy from Grainger, and in some places they are pretty liberal with cash sales.
If I had to install such a system, I would go with a VERY LOUD alarm; but I don't have to clean up the mess.