How do I make connections with medium to large advertising agencies?

I own a design agency that offers very high quality graphic and web design services. Up until now we have been more of a B2C business, offering logos, brochures, direct mail, newsletters, websites, etc.
We work with a couple advertising agencies that outsource their creative needs to us. It has turned out to be a great asset to these companies because they can pick up the phone and call whenever the need arises and take comfort in knowing we can handle whatever they throw our way.
Currently, we are changing our focus to be more of a B2B design agency. Up until now, all of our B2C business has been provided through word of mouth referrals. I would really appreciate any advice people could give me to help open some new doors to advertising agencies in need of outsourcing solutions. I’m not talking about outsourcing to India either…our talent is 100% American! Thank you in advance!