There are many websites from where you can download the CSIR NET Model Papers.
But, all the sites are not providing good quality in their mock tests. So, I would like to suggest you to choose it wisely.
You can get these test papers for free on the official site as well as on other sites.
NTA has also started a portal where you can attempt the mock test paper to know about the online examination pattern.
With the mock test papers, I would also suggest you to solve last few years CSIR NET Question Papers. These papers will help you to know about the type of questions that will be asked in the exam.