The FETs seem to be being used as switches but the description says they have to be matched. This is because they are all being controlled together.
This might mean you would have to buy 10 of them and test them to get similar ones.
They don't say which way they should be tested but one way would be to test them in a multimeter that has a transistor tester in it. This only tests for gain, but it may be enough.
In this circuit, it might be better to test for cutoff voltage, but you would have to set up a bit of equipment to do this.
This is the negative voltage on the gate that just stops the current flowing between the drain and source.
TWO 9 volt batteries
one potentiometer, about 10 K total resistance.
TWO multimeters, one on voltage and one on current.
See attachment. Be careful to get it right if you wire it up.
The FET gets negative voltage from the potentiometer across the left battery and it gets its normal positive voltage from the battery on the right.
Current is measured in the drain lead of the FET and voltage is measured across the Gate- source .
You need to check a few FETs for
(1) gate voltage to cut off drain current and
(2) drain current with zero voltage on the gate and
find 4 that are most alike.
Just to remind you of my earlier comment:
There is always a risk that a substitute transistor may not work, so try at your own risk.