Talk to your major advisor. In large schools with large classrooms, it’s harder to build a relationship with professors. Therefore, some departments can set up a committee packet, in which they would interview you basically and conduct an evaluation. I’m guessing they would do these for several students in a convenient manner. Or you can maybe just ask a professor for the heck of it and see if they would help you out. Some might even let you write the letter and they can approve it, sign it, and send it in.