I mostly shop at thrift stores aside from things you'd want to be new, like underwear, etc which we've gotten Fair Trade online. I like American Apparel, especially the extensive online reviews for each item.
I also quite a bit myself, often from re-purposed fabrics - like sheets, adult sweats and felted wool sweaters all make great kids clothes.
Thrift stores are great because the price is usually low, discarded clothes are not going into a landfill and even ethically-made clothing uses a lot of resources, but thrifting doesn't. I also stick with only natural fibers and hate logos and most graphics on clothing, so that makes finding things a bit harder.
SAHM to two amazing boys