This is where I have bought steel balls before. Good quality, no flats. Price includes shipping:
http://www.trulytexas.com/bb-1-2.html
http://www.trulytexas.com/bb-3-8.html
I just received some 1/2" in the mail today actually. Their 200 rounds weights a little less than four pounds. Was it 3.8 lbs? I weighed the incoming box with contents (USPS small flat rate box), but I didn't write down the exact weight. I'm pretty sure it was 3.8 lbs though.