You can definitely prepare for CAT by studying yourself. First of all get the previous year papers and go through the questions. Once you get to know what kind of questions are asked you can prepare that way.

For Quantitative aptitude section the book of either Arun Sharma or Sarvesh Verma is good.

For English try to read daily newspapers especially The Hindu and if possible read The Economist magazine also because many a times the Reading Comprehension passages are taken directly from there.

For DI/LR past year questions are sufficient.

Moreover you can secure excellent scores if you get some test series also because they will help to manage your time.

All the very best