I have a Yard Machine 16.5 hp lawn tractor (MODEL 42A707) that will not start unless I use jumper cables and jump start it from my car. The mower will run fine while I mow my yard but as soon as I turn the mower off I cannot re start it unless I jump it from the car again. I have replaced the battery and spark plugs but this makes no difference.
I appreciate any advice/help.

I wonder if you are losing your alternator/generator voltage-perhaps the regulator is dead.

Start your mower and using a voltmeter/multimeter check the charging volts by removing the positive from the battery and then testing between this and earth/body there should be around 14.5 volts coming down the cable. (unless you have a 6volt battery, then you should get 6 to 7 volts of charging voltage.)

Bear in mind I'm not totally familiar with this mower.