It is correct that LSAT passages are tougher than the GMAT passages. There is a book called Powerscore LSAT RC
Bible. I haven't gone through this book but I have heard good things about the material.
As far as tough RC passages are concerned, you can always check out the RC section of the GMAT club forum. There are few tough (LSAT) RC's being posted by few members almost daily..!!
In my opinion, that would be sufficient along with the OG stuff as far as RC is concerned.