For Testbook website:
Test series are provided according your exam, hence to find them you can follow this path > SuperCoaching > right of Dashboard > Test Series tab.
We are also enclosing the screenshot of the same for your reference.
Also, On the same page if scroll down a little, you'll find options like 'All mocks' section including unenrolled tests and can be added with (+) button, search for a particular exam test and previous year papers are there as well,
Please check below image for reference.
For Testbook App:
Here the path is same here just go to your SuperCoaching section > right of Dashboard > Test Series tab.
Here you'll find all of the Tests including previous year papers. Please refer below screenshot as an example.
Hope it helps, Happy Learning!