Some of my favorite pairs of pajamas came from walmart and I have the entire set of Sesame Street underwear from there, They have a great selection of childish wear for adults. They come in larger sizes too. However I admit that most of my footie pajamas come from K-Mart, joe boxer makes some awesome footie pajamas and they go up to size 3x when they are in season and usually go on sale for Christmas.
I have lots of footie sleepers from there too!! They do great in the winter!! I wish I could get a little smaller and be able to buy 14/16 at walmart, however I do splurge every now and then at Justice for girls, they have a 18/20 size in clothing and I love it!