It is not hard to break into this market. Start with your local franchise and corporate store managers; they will generally give you contact information for their corporate support departments or sign suppliers (most corporate owned and franchise stores are contractually obligated to order signs through their corporate support channels).
These large sign companies are constantly on the lookout for qualified sign installers and maintenance service. You can also advertise in Signs of the Times. Generally, once you have been vetted, they will send out survey requests and ship signs directly to you for installation.
It's good work if you are set up to handle it. Usually it helps to be a Union shop and have good references. They can be slow to pay, but that can be alleviated by having a good contract and an understanding of their business model. These sign forwarding companies usually release funds after they have been paid by their clients (they are not banks). You are not a bank either, so you must price accordingly (you may have expenses that you need to put on the books for months). Most everyone I have worked with involved with corporate accounts are understanding and helpful, but you cannot give away the farm. They will try to lowball you, but you need to stick to your guns and pass on opportunities that don't support your business model.