I think GMAT is quite easy, and you can prepare for it yourself. You can start with the Official Guide, 11th edition. Then if you feel you need more practice in a certain section, you can buy the Quant/Verbal Official Guides. Then, to round up your prep, you can get the Kaplan 800, which has the toughest level of quant of all GMAT prep books. I think if you do all this for about 2 months, you can easily target 750+.
Also, make sure you solve lots of practice tests. Do solve the 2 free tests available on GMAT.com(GMATprep). Believe it or not, the score you get in these tests after you finish your preps will be very close to the actual score you can expect to get in the main GMAT test. For example, I averaged around 730 in GMATprep, and I would have got that, if not for silly mistakes/nervousness.
I had around 7 days to prepare for GMAT, and I got 680, and I'm disappointed that I couldn't cross 720-730(I messed up a couple of easy quant questions).