Apart from GMAT Club (read others' experience and they have 2 CAT practice tests for free too), I use these books.
Sentence correction - Manhattan (well written yet the questions aren't enough for practice)
Critical reasoning. - powerscore
Since my math background isn't strong, my main focus is to build up from the fundamentals.
- I use 5 manhattan books
- Jeff Sackmann's questions
- official guild 13th and the quant review.
I am taking a course online on Magoosh and soon with Kaplan. I think the key to success is knowing the concepts well and tons of practice. If your math and verbal skills are great already I don't think you need to take a course.
Good luck