ebay from the store MsL Big Tots.
you can try Forever A Kid Sleepwear For Boys & Girls
It's pretty dang pricey, but they're customizable which is nice. I haven't purchased from them myself, but I would really like a pair of kids pj's too and those polo pjs are awesome!
I have lots of stuff from MsL Big Tots and I love them! But they are fleece and I would like to get some polo pjs that are tshirt cotton with the ribbing... Might try making my own