When you say "alerts", that could be a few things. One could be someone who sets up a specific scanner so that they get alerted when a stock fits their set criteria such as volume or price alerts. Then there are promo alerts where a group picks a stock to pump and alerts members prior.
If you are looking for a market scanner, I can only tell you what I use and it is Strategy Scanner by Etrade - u have to be an active trader with them in order to get it.
As far as promo alerts, I would never recommend any of them. I don't use any of them. Only thing I will monitor is if someone is promo'ing a certain stock by checking twitter.