You can change your pressure switch to a low pressure cut off type switch. Square D makes one. It cuts the pump off if the pressure falls to 12 PSI. That will be the simplest, cheapest way to do it. If the cut off is activated, there is a lever on the switch that you have to use to reset the switch.
I have the "low pressure cut-off" version of the pressure switch at the input of my ballast tank that controls submersible pump, as I have house, barn and a few apartments on well and always feared somebody would leave a faucet on, or a pipe break, etc.. when not home and pump would burn up. ...
The down side is if the power goes out for an extended period of time and too many people flush toilet etc.. and reduce pressure to below cut-off point. When the power comes back on you're still without water until you manually hold reset lever on switch and pressure builds back up to above the cutout pressure. For me, that means climbing down through an outdoor manhole into a dark cramped underground vault. And if I've been away at work all day or on vacation, nobody has water until I get there.
It's like paying for insurance.
If you go with float switch wired directly in the wire between the pressure switch and pump, make sure it's rated for 220V and
horsepower rated for the motor. That is, switch is not just rated for motor's full amps, but can handle motor overload and motor starting amps too. Generally a (general use AC/DC) switch's ampere rating should be twice the motor's full load amp rating if a horsepower rating is not given. [For an AC (only) general use switch, 125% or greater of the motors rating is ok]
If wired directly, float switch should be 2 pole, that is, it interrupts both wires / legs (L1 & L2) of the 220V circuit and not just one leg, which would leave voltage through the motor even when off.
If float switch operates (drops out) a relay whose contacts interrupts pump's power circuit from pressure switch, relay's contacts should be rated for pump hp/amps as mentioned above. Relay (coil) & float switch will then need to operate on the separate 120V circuit.
(Note: there are ways to operate the float/relay off the existing 220V circuit from the pressure switch, but that gets more complicated. That is, you either need a (white) neutral conductor (in addition to (green) ground conductor) run with your 220V so you can get 120V for the relay coil and float switch, or you have a 220v relay coil and float switch. Also in these scenarios, the relay would be
starting the pump
every time the pressure switch calls for water rather than just
stopping the pump in an low water situation.)