Ebay as well as oldnavy.com. They quite often have great sales.
Zulily.com has a bunch of cute boy clothes for cheap my mom has ordered numerous 3 pieces from there for like 10-12 bucks. (:
I got a whole lot of cute stuff on eBay. European stuff, all pretty much unworn. (You can even specify new with tags if you don't want anything that is used, no matter how lightly used it is.) Got such a cute onesie there, yellow with a big eggplant on it, things like that.