where to study CR from?

there are very few sources of verbal problems that consistently match the official ones in quality, method, and style, so it's best to maximize the utility you can derive from the official problems.

for CR -- one thing you can do on strengthening, weakening, "evaluating the argument", and "explaining the situation" problems (which together account for at least 80% of the problems you'll see in CR):
make a new correct answer to the problem.
in other words, make up a new "answer choice" that doesn't work like the original correct answer, but that would also be a correct answer if it appeared in the choices.

here are a couple of examples, using problem numbers from the 12th edition OG (i'm not allowed to reproduce the problems here):

* another possible correct answer for #109: most prank calls to the fire department are made by teenagers, whose use of private telephones is monitored by their parents. (strengthens the argument by providing additional justification for the idea that prank calls will go down if people are forced onto private phones)

* another possible correct answer for #65: no modern history of cyprus mentions any earthquake near the island other than the one that occurred in AD 365. (strengthens the argument by eliminating, or at least rendering much more remote, the possibility of another earthquake that might have caused the observed destruction)

if you do this, it will help your brain start thinking in the way it actually has to. (obviously there won't be an "answer key" for an exercise like this one -- but, if you are starting to get the right kind of handle on the way CR works, you should have a pretty good intuition about whether your creations work.)