I'm using tdameritrade and scottrade. Entering orders the normal way usually takes too long in fast moving markets. Going through their website and calculating how many shares to buy usually takes from 10 up to 30 seconds. Does there exist a programs where you can store some "ready to buy" orders, the symbol and ticker ready, which can be executed at the click of the mouse?
Sorry for this late reply, you may have already found what you're looking for. If not, check out client side applications offered by most direct market access brokers. Many applications offer hotkeys or pre-loaded orders that can be sent really easily. Check out www.protradefinancial.com as an example. Good luck finding what you're looking for.
One thing to try: You can enter a limit order way outside your desired price (like, if you want to buy 200 shares of LEH today at around $40, put in an order for 200 shares at $35), let it sit there not getting filled, and then change the order to the desired price when you want. Ameritrade's Command Center has a fairly quick-to-use trade ticket window.