Hang out at popular concert venues and snap pictures of the band for free. Try to make them look cool. This works better if the band is not widely known. All that they want is publicity. Get the band's information and show them your best prints. If they like them, they will ask you to photograph them again.
Once you have this offer, capitalize. Give the band pricing options and your information. Sell them the prints you just showed them. Make sure to give them more than one of your cards. Word will spread from band to band and you want each of them able to give out accurate information.
Hang out with the bands you photograph. Make friends and become a part of their community. The better you know them, the more likely that they will refer you.
Along with advertising directly to bands, leave flyers around town in places you think bands hang out. Concert venues, music schools, bars, clubs etc. Make sure to include your best shots on the flyer so people know what they are getting.
Talk with the clerks at these places. Give them some of your cards and offer them a finder's fee. Your offers will skyrocket.
Don't give out your cards to random people. It is a waste of paper with no likely benefits. Place your advertising in places that the people you want to see them go.