Krishna, I recommend that you do buy these practice questions. They helped me, and it's not very expensive.
But before you do that, I think that a structured training program is really necessary, or at least it was for me. There's a lot of material that you need to know. Unless you're in the trenches using it every day, the breadth of all of it seems overwhelming to me, and a training class will take you through all that, making sure you cover everything that's important without getting sidetracked on the stuff that's not part of each certification.
I recommend using the ACloud.Guru training, like this one: https://acloud.guru/course/aws-certified-developer-associate/dashboard (I hope that link works for for you - since I bought the class myself it brings me straight into it, and I'm not sure it's the same for new customers). Also, look around a bit. You can save a bunch of money on a bundle of courses for multiple certifications, or if you search the web you can probably find some discount codes.