The Collector ponies were at Walmart. The others probably aren't at Walmart because Walmart chose not to pick them up. It's not Basic Fun's fault. They can't just burst in and leave a pile of ponies on the floor. The store needs to order them.
As far as the 1st set of Rainbow ponies go, they came in boxes of six, one of each character. Moonstone isn't any rarer, but maybe her color scheme was the most appealing to shoppers in your area. With the 2nd set of Rainbow ponies, they also come in a box of six, with Tickle x 2, Confetti x 2, Flutterbye x 1, Trickles x1. (But I assume it will switch to "one of each" once they release Pinwheel and Starflower.)