www.brokerageamerica.com might be a good one. being from Canada i cannot open an account with them.what i would warn u to do is always call and ask if u can trade unlimited amounts of shares per order. tradingdirect.com on their website says u can trade unlimited shares but will only let u trade 500,000. aftrader.com will only let u trade 250,000.
not sure what the minimum with www.mytradz.com but they will let u trade 2 million shares before tagging u with more.
lowtrades.com has no minimum but they won't let u trade an unlimited amount of shares any longer. maybe 500,000?