You can buy Kaplan 800 or u can also buy BTG questions

Be careful to use good questions! Critical reasoning questions are hard to write - much harder than quant.

You have done lots of official questions by now so you should be able to "smell a rat." If you feel like a question is not fair or is not like official questions then trust yourself or else post it and ask about it.

Whatever source you use probably some will be good and some will not be. But as you are looking for hard questions, well those are even tougher to write because they have to difficult yet still fair. Many supposedly hard questions are either not difficult, or not fair. (Not fair would mean either no correct answer or else more than one).