If it is for a college course, I'll second what someone else said and mention that sometimes they have them available to be checked out and used for the semester.
Also, as someone already mentioned, be sure to put your name and contact info on the calculator! They get left in lecture halls and labs all the time, with no name on them. As much as people would like to get them back to their rightful owners, it's impossible if they aren't marked! So be sure your name is on it, both inside (battery cover) and out (main body of calculator, not just the removable cover).
Lastly, it seem the TI-83, TI-84 and TI-84 Silver all do pretty much the same stuff. In all honesty, I've seen TI-82 and TI-81's that are still in fine working order. The only issue with what you need is whether a certain college course perhaps demands a particular type. For many courses any will do!