while the Yellow bags have been available on eBay overseas and through some Toy retailers for a few months, we've yet to see them in USA stores. I'm hoping to see this change in the next few months. Select retailers had a FEW in over Christmas, and that's likely to be just the tip of a coming influx. So, to get ready to find them all in stores, I've set up a handy ID graphic to help you spot the ponies you want in stores!
It's also worth noting that the Princess blind bag figures (the Alicorn pose) is much larger than the others, and they HAVE packaged them with an extra plastic wedge to keep those extra long wings from bending. If all you want is to take home a princess, look for that extra bit of weight and that crunchy crackle inside the bag to know you've picked a winner. :)