I keep on hearing how it's better to take those because they build up your endurance but I have no idea where to get them from.
Be careful how much you let such comments affect your studying. The "R" versions you are talking about I assume are the
revised versions of the exam that AAMC released back when they made a change in the sequence of the MCAT (PS became first and VR second) and reduced verbal from 65 questions to 60 questions in the paper version.
Once you get a hold of these magical exams and see that all they are is the same exams available to you on line, but with a few more passages, I think the hype will subside. Just do the AAMC CBTs and you'll get exposed to exactly what the "R" exams will do, but it'll be in a better format.
I will agree that the "R" exams can prove useful for a second time test-taker who has already used all the AAMC CBTs, because it will give them ten new passages per R-exam. But keep in mind that not all of those exams are real exams. For instance, AAMC Practice Exam 1 was written in 1990 as a sample of what the
new MCAT was going to be. It was generally regarded as unrealistic and it gave a score that was lower than expected. People saw big jumps from AAMC 1 to their next practice exam without doing much studying.
I hope SN2ed weighs in on this, because more than anyone at SDN, he has kept in tune with the MCAT over the years.