Your department head and university financial aid officers are good sources of information about scholarships for which you might be eligible. But it doesn't hurt to do a little searching on your own.
Be careful: Many so-called scholarships are little more than marketing schemes. And beware of any scholarship that requires you to pay for a chance to be considered.
There's no one central clearinghouse of all the legitimate graduate scholarships and fellowships, but here are a few reputable sites to try: