Go to home depot or lowers or any carpet store, but make a measurement first so you could tell them exactly how big you room to be carpeted is. If you don't know how to measure, they will tell you how. If still it won't work, you could tell them if they could go to your apartment to do the measuring (but you have to pay extra fee) for you. You can install it yourself - get some nice padding first. or you can hire someone to install it for you.
Additional... Sorry, the paragraph below your question did not appear before I typed in my answer...Anyway, you could go to the copy of your contract and line per line, read the contents. If it's not there... you have no choice but put a new carpet before you move out. If it didn't say how much or what kind of carpet as long as you are carpeting it, just put a cheaper carpet and a cheaper padding. Anyway, your the one that used the old one. Brown carpet is always a good colour for an apartment. Or you could request a new carpet...Try everything you could first for them to put a new carpet, if nothing works, just put in a new one.