Hello Ashray,

You can register for the mock test of JEE Main from the official site of NTA. The registrations will be available on weekdays and the candidates will have to visit the test practise centres to attempt the mock tests which will only be active on weekends.

And, yes the registrations for mock tests for preparation of JEE Main 2019 is completely free. You will not be required to pay a single penny for the same.

All the best!