I love Once Upon a Child for my son's clothing. It is used clothing, but the ones near us are name brand, like new stuff. I often find items with the tags still on it. It benefits local families, by allowing people to sell their kid's stuff back for cash or store credit. It's so packed with clothing, there's no way you are "stealing" anything from anyone else. The more you buy, the more they can buy from others to re-stock.
For never been owned clothing, I like Old Navy and Gap for both my son and I. They have decent sales, but not typically what I'd consider cheap. I tend to buy a couple of pairs of cotton pants and cotton shirts at Baby Gap at around 2 for $20, every season. The quality and style is always great and I use the new stuff to supplement the second hand majority of his wardrobe. We also do Kohls brand Jumping Beans. Its cheap on sale, lower quality than Baby Gap, but still reasonable.