You can save money by piecing and AR15 together (lower/upper), but its getting difficult. Putting an upper an lower together takes all of 10 seconds.

I know Midwayusa.com you can buy a few different makes of lowers, and you can buy uppers there.
Ameetec.com
adcofirearms.com (I've used Steve here 3 times for ARs and his service is excellent)

Hope this helps ya out some. You might also want to check the classifieds at ar15.com