dear student,

You are required to clear two papers.

You will have only maths questions in the first paper. The sample questions given on the institutes’s website are actually a very good 'sample', you will have very similar questions in the exam. I do not know of any other good resource where you will find similar questions. I searched for one when I was preparing for the exam but failed to find any. I agree that the questions are tough but then they will be tough for everyone, and so, you can expect the cut-off to be very low.

The second paper's preparation would depend on your academic background. Again here, there will be some maths questions which are compulsory and then you can choose to answer questions from one of the Maths, Statistics, Physics, CS, Electronics sections of the paper. In my opinion, for this last section, you can prepare using the GATE material for the corresponding paper. The biggest difference here is that the questions will be subjective-type unlike GATE.

EDIT : There is a book called "Test of Mathematics at the Ten Plus Two level". Many people call it the TOMATO. It contains maths problems that can help you to prepare for the MMA paper.  Its a book published by the Indian Statistical Institute and is available on flipkart.

Hope this helps!!