Every working artist usually makes it pretty easy to contact them or a booking agent/manager. Look at their website, facebook, etc for a booking email or other contact form and approach it professionally. Sell the project like you would to anyone else you want to be involved. If you can pay, offer a standard license fee or similar, if you can't afford that, ask for something less. Being independent, they may be a lot more likely to sign on for a little less or in exchange for some prominent placed "featuring music by.." on posters, DVD covers, etc.