The reliable source is the Arihant Guide. You will be provided with total 6 practice tests, 3 in CD and 3 in the book itself. These are the key to success in BITSAT. Without any disturbance, by creating an actual exam condition, within a given time, just solve all these tests after completing the book. You will definitely be unstoppable in the BITSAT after that. Just gear up your time and rock the paper.