I think you really should invest in some of the
MGMAT guides. I think that you should, at least, get the number properties, word problems, and Sentence Correction guides from Manhattan.
And study these books. I would spend hours just studying the books and concepts and going over and over them until they are solid. Do the practice sets multiple times and understand the process of getting to the correct answers.
But the GMAT is more than just math, it is about thinking about concepts and how they relate to other concepts. You can do very well on the GMAT if you have a strong grasp of the foundations.
Borders is going out of business in many areas and are having massive sales on books. Might be able to get a these guides at about 10 bucks a piece if you go to borders. Otherwise order them.
Also, try your public library. There's a chance they might have them. but you need a guide I think.