Rather than replace the feet, I replaced the bottom case. It cost about $25 for a lightly used one on eBay (“Grade A-”). Replacing the bottom case may seem extreme when you just want feet, but it does seem like the way the part is manufactured the feet connect from the inside to the outside, so it’s actually not crazy. I had gone to the Genius Bar for this (where I learned their approach to missing feet is to replace the whole bottom case), but they quoted $180 total ($101 for the part, $79 for labor). Too much for me.
I believe the exact Apple part number you want for 2012 and early 2013 Retina MacBook Pros is 604-3590-A. This fits my 2012 rMBP anyway, and listings for it say it fits early 2013 too.