My boyfriend is a painter. A lot of their business comes from referrals, including when a paint store they purchase their paint from, tells a customer. Some look on websites, but not all contractors have websites. You can ask friends on Facebook and there are some sites that offer a free search, but those may not be represented well when the contractor is paying to list on that site. Ask Angie is a paid site for customers. Hardware stores, some are good bui if there are nothing but young, inexperienced people working there, they cannot help you.
Always best to check with neighbors, places that sell to the contractors and check the Better Business Bureau to ensure quality business.
Never pay in full, up front.