Little Lords and Ladies -chantilly
http://www.littlelordsandladies.com/
Buy Buy Baby -- springfield
http://www.buybuybaby.com/default.asp?
kid to kid -- centreville
http://kidtokid.com/
There's another one in Manassas, but it kind of sucks.
freecycle
We get lots of good free stuff from there and give lots of free stuff out
http://www.freecycle.org/
craigslist -- tons of stuff there
yard sales -- the best place to get stuff super cheap.