I've spent quite some time since I started submitting stories attempting to find suitable markets. There are very few professional SF magazines, quite a few semi-prozines and plenty of small press and webzines, and they all have slightly different audiences.
You can get information on them from such websites as Ralans and Spicy Green Iguana. If you want to submit to any market, you need to read their submission guidelines every time. They may have changed, or they could be on hold so you really need to know your market.