Personally I do not have a lot of experience with nootropics. There are a lot of bulk powders and such I am sure people will recommend for you. However, I have used Lucidrol out of our AMS lineup and liked it quite a lot. Really helps me focus and provides a sort of tunnel vision on what I am focusing on for me. Nutra has it for 17.99 right now I believe.
Best of luck on your exam