The best way to find ad networks can be through your trade association or by asking business contacts for recommendations. They may be able to give you the names of agencies with experience of working for your kind of business. You could also contact the advertising departments of trade publications for your industry and ask which agencies they regularly deal with.
If you have a choice of several ad networks, you need to decide which one suits you best. Even if you already have one preferred agency, you can use the same process to help yourself focus on what you are looking for and make sure that the advertising agency understands your needs. Chemistry helps too.
Start by drawing up a brief, explaining what you are trying to achieve. The brief should include background information on your business and the types of customers you are trying to reach. The more specific you can be about your objectives, the better.
Circulate the brief to the ad networks on your shortlist, and ask them to come back to you with a proposal including recommendations for where you should advertise and how much you should spend. Make sure you know how much they will charge. Agency fees typically include an hourly rate for creative work and a commission on the value of any advertising they buy. Check that the agency will pass on media-buying discounts to you.