Oh yay! I get to teach WS something. Doesn't happen often.
R positions are new creations by the NRMP. They are "advanced" spots that start the year of the match. They are designed to accomodate the "all in" rule. If an advanced program is "all in", then all of their spots need to be filled in the match, even those 1 year off cycle. So, if a program matched a candidate in an advanced spot in last year's match, and that candidate couldn't find a prelim / changes their mind and gets a waiver / etc, that program must fill that (now empty) position in the match. But, they now need to fill a PGY-2 position for 2014 in the 2014 match -- so matching another Advanced doesn't work. So, the NRMP created a new code "R" for PGY-2 positions in the match starting the year of the match. Candidates for these position must have already completed the prerequisite training.
Now, back to the OP's question. Where are these positions?
1. Remember that these are NRMP positions. You're not necessarily going to see them in ERAS. Programs are likely to simply review their ERAS applications (for their advanced spots), review to see if you would qualify for an R spot, and talk to you about it when you interview.
2. These are new. I don't think that all advanced programs feel they need to use them, and they may simply continue to offer them outside the match. Whether or not that's "match legal" or not is a debatable question.
So, the only way to know if a program will have a position like this is to contact them directly.