Craigslist for used ones, but you have to be careful because if it was in an accident, they say not to use them again.
Otherwise, our county does a car seat safety check twice a year. At the safety checks, they also sell car seats for like $25, which is really cheap for a new seat. Check with the health department to see if they do it where you live.
at 1:38 AM on Jun. 16, 2011