Most of what people wrote, except for Alaska Mike, applies to the country and pop side of the music business, not to folk music. Most folk musicians, from the least known all the way up to the biggest names are a lot more accessible than that, and most can be approached directly, as long as you don't bother them when they are working. Anne Feeney, the labor union songwriter, pitched "Have You Been to Jail for Justice?" directly to Peter Yarrow, and now the song is on PP&M's latest CD. I strongly suggest that you speak with the artits first by e-mail, snail mail, telephone, or in person, and don't send an unsolicited demo. Contact information is generally available on their websites.