Dear Student

There is not any single source where you can prepare but there are multiple ways through which you can prepare.

you can buy previous year question paper book for preparation of NDA written exam.

paid sites are Grade up and unacademy

For free sites you can take help from YouTube.

you can go through the website of Vidya Guru online coaching, there you will get the whole NDA exam preparation online course.

For further query feel free to ask!

Hope it helps!