Can't say I have all of the experience everyone else has in roofing, but I have a lot of experience in business.
I agree with Jim. Make a call first, but DEFINITELY call around. People are always looking to add good quality subs.
Don't lead your conversations with price. If you lead with price, that's all you are. A dollar figure. Lead with value.
Call ABC Roofing (made up company), ask to speak with the production manager or owner. If you get one of those, do as Jim said. Tell them you want to grow your business, and that you want to help them grow, too. Ask them if there is a time you can meet with them and try to set an appointment. If you get an appointment, go to it prepared with pictures of your work and safety measures, insurance and business information, and be ready to discuss pricing.
If they don't readily set up an appointment with you, or a receptionist or otherwise says who need to speak to is unavailable, ask for their email address so you can send some examples of your work. Send them before and after pictures, pictures that display you are serious about safety. Tell them you have your insurance ready and available.
Companies have crews they are used to working with, so make sure you let them know you aren't necessarily trying to take someone's place. When you say you want to take another person's place, it's hard to lead with value. You end up just having to beat someone else's prices. You just want to be an option for their company when the time comes. And if I learned anything from being a vendor, THE TIME WILL COME. They will need you one day, and that's you time to shine.
Anyway. That's my two cents on business as it pertains to vendors/subs.