The best sources are official sources. Maybe look at the challenging problems from question pack 1 on the GMAT.

800 level questions??

There are no 800 level questions. Only 1 person in 4000 scores an 800. Getting an 800 is a matter of consistency. I earned a perfect 51 on the verbal section - meaning that I likely did not miss any scored questions - and I did not find anything unexpected or really any more difficult then what you would see in the later parts of the Official Guide or for example in the Veritas Advanced Verbal book.

I would say therefore that the key is to get questions right at a very consistent rate and in the appropriate amount of time.

If you really do want to practice some harder critical reasoning and reading comprehension questions you could always try out some LSAT questions. Here are a couple of articles I wrote about using LSAT questions. Personally, I try to make sure I can do questions at all levels and I think that those in the official sources and specific GMAT resources like the new free Veritas question bank are tough enough!

https://www.beatthegmat.com/using-the-ls ... 66548.html
https://www.beatthegmat.com/lsat-to-stud ... 69915.html
https://www.beatthegmat.com/lsat-to-stud ... 71361.html