when you say there is a place to put it i'm guessing you mean one of those plactic things that clip onto the front on the inside of the case. the thing that the pc speaker is normally attatched to and where you can put a case fan if you wish.
in that case what you want is probably a 80mm fan, thats the standard size, but you may want to measure to be sure. you can spend anything you want on these fans, it all depends on how much added cooling you want AND on how much noise you can handle. any computer store (computers store, not business depot, etc) should have a decent fan for $5-10. these fans dont move a lot of air but you also probably wont be able to hear it.
when you get the fan make sure that the plug on it has a pass-thru molex connector. this way you will un-plug a harddrive, plug that into the fan, and then the other fan connector into the hard drive. that may be a bad explaination, but it'll work.
if i was wrong and you dont have one of those plactic things in the front of your case and your just talking about a grill area thats cut into the front:
if theres no plastic thing and no screw holes you'll have to drill some and use a self tapping screw you should be able to get at the comp store (at least at the one i go to)
if you let us know more about your computer and what you want to do with it we could reccomend other things for you as well.