I don't know of a source that provides ONLY 700+ questions, unfortunately.
I have a question for you: is it the case that you can already answer 95+% of the sub-700 level questions correctly
and at least 30 seconds faster than the average time limit for that question type?
If not, you're not done studying those sub-700 questions. The material on this test builds. Part of how you learn to do a 700-level problem is by mastering lower-level problems of the same type. If you can answer a 650-level quant problem correctly in 2 minutes, that's great - but if you then want to be able to answer 700 or 750 level problems of the same type in 2 minutes, then you'd better have figured out how to do that 650-level in 1.5 minutes. The 700+ problem is going to require you to be able to do all of the stuff in the 650 problem PLUS more, and you still have to stick to that 2m timeframe.
You've worked through a lot of material already, and some of those were 700-800 level problems, right? Have you mastered all of those? For more on what I mean by mastery, here's an article about how to analyze practice problems:
http://www.beatthegmat.com/a/2009/10/09/how-to-analyze-a-practice-problemHave you analyzed your existing 700+ problems to that level of attention? If not, you're not done with those problems yet, and it's not worth your time to seek out new problems until you've learned everything you can from your existing material.
Because you're asking about 700-800 level problems specifically, I'll assume that you are aiming for at least a 700, if not higher. You may also be interested in this article:
http://www.beatthegmat.com/mba/2010/04/15/the-distinction-between-a-700-score-and-a-760-scoreThat article is valuable even if you're not going for a ridiculously-high 760. It gives you an idea of the kinds of things that a 700 and a 760-level scorer can do - and it's not really about doing tons of new problems all the time.
Take a look at those articles and let me know what you think.