R.D.
I like Children's Orchard. They buy 'gently used' clothing and resell it. They always have good quality, name brands, as well as 'play clothes' (may have a very small stain or button missing) at a lower cost.
Garage sales, Craigslist, eBay, and consignement shops have already been listed, and I agree with those mamas.
There's also thredup. Use this link as an invitation (http://www.thredup.com/228757 ) and when you sign up, you get a $5 credit! You can buy, sell, and swap BOXES of clothes with other moms around the country. (Invitation is open to ANYONE that wants to join! You can ALL get the $5 credit if you want!) :o)