You may also consider partnering with a medium size or boutique agency that has experience. Do a joint venture agreement or revenue sharing agreement so that you have a credible outlet from which to offer your product.
For example, I work with a company right now that is helping search engine optimization companies and web design firms increase their cash value per customer by offering something they don't even have to service. This makes it a lot easier for them to offer it to their clients, remonetizing their database and increasing future cash value per customer simply because they can offer a much more complete package to them.
Find out which advertising agencies are local to you and take them to lunch or coffee and give them a short explanation on what you're thinking about doing. If they seem interested, see where they can help take it. Be sure your agreements with them are clear and take it to the bigger companies through them.
Related: Building a Successful Advertising Plan
Related: Advertiser and Advertising Agency Contract