If you sing in an a cappella group, or would like to start one, you're gonna need a few good arrangements. There are books of arrangements out there, plus arranging services and listening tapes, etc.
And now there's a website, started by a Michigan undergrad, called Better A Cappella—which offers a database of information about existing a cappella arrangements. Does your group want to sing "Accidentally In Love?" I don't know why you would, but let's say you want to! Go to Better A Cappella. Search for the song title. Find the contact info for a dude with an existing arrangement. And shoot him an e-mail.
Of course, doing your own arrangements is a good learning experience. But for those short on time (or ear training), check out Better A Cappella.