Here are some sources where you can find some tough questions:

Quant:
1. Manhattan GMAT Challenge sets
2. EZ solutions Advanced math workbook
3. And if i'm allowed - the Gmat Club tests.

Verbal:
Now this is going to be tough as there are hardly any sources exclusively for tough questions. But LSAT material for CR is generally considered tougher and good for practice.
Also MGMAT practice tests and kaplan practice tests offer some challenging verbal questions.

There is this Kaplan800 book which claims to be for the 800-score aspirants - but the quant section in the book was not worth the 700+ level. Verbal was much better especially the RCs..

And finally, you can find some really tough GMAT questions right in your own backyard - I'm not kidding - the source is GMATPrep. Do it several times and since some questions would repeat you will get them right and will face the tough upper bin questions. I have done the GMATPrep 4-5 times each. And during the 4th and 5th time i came face to face to really good tough problems and questions..

Hope this helped.