Hello. You can download sample papers and previous years question papers for Jee Mains free of cost from Careers360 website. The link for the same is provided below.

https://engineering.careers360.com/articles/jee-main-sample-papers

I would recommend that you practice these tests after you have completed the syllabus and also increase the frequency of solving the sample papers as you reach closer to the date of exam. 

For April session of Jee Mains 2019 you can check the below link for exam preparation,mock test series, tips etc.

https://www.entrance360.com/knockout-jee-main-april/

Hope this helps. Good luck!