You really want to stick with official questions for Verbal. Math is universal and has clear rules that are accepted everywhere. While GMAT Verbal follows many generally accepted rules, it also has a style of its own, and the test writers determine which rules they will enforce and which they won't. Quality verbal questions are hard to come by, and people who study from sub-par questions often learn the wrong things and get confused!
So I'll second the recommendations for GMATPrep above. You can also purchase other official materials, such as older editions (the 10th ed. OG is good and available second-hand at low cost) and the mba.com "paper tests." Given the cost of a GMAT retake, a few bucks on quality questions is a worthwhile expense.
Having said all that, I also recommend that you work back through the OG. Most people are too quick to look for new material without truly mastering what they've already covered. The OG should really be all you need! For instance, in SC, can you explain all the issues in each question? Can you find the quickest way to get to the answer? Can you eliminate multiple choices based on just one issue, such as subject-verb or meaning? In CR/RC, how often can you prephrase the answer before looking at the choices? Can you see why each answer is wrong?
I have some more suggestions in this post:
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