It varies from state to state and from program to program. For some private schools, they do not even require it or they just want you to tell them what score, but it doesn't have any weigh on whether you will be accepted or not.
Yes, going to places like Kaplan will help. They basically teach you certain reasoning methodlogy to tackle questions asked from the exam. The classes are kind of pricy, but you can also purchase their study guides.
To cut down some cost, you can go to halfprice or bargain book store. Sometimes they will have GMAT or GRE prep books for much cheaper price. They may be an edition or two behind the most current ones , but the material is essentially the same.