Great business! Why isn't sustainable? This is a great business model!

Business dev and sales are often (if not always) the same thing.

What you need is to hire cold callers, hire one or two sales people team and pay based on commission or base plus commission so that it saves you some money. You have a great business here and just need to be creative in growing it.
Growth back it! Hire sales, make your own ad line, have dedicated markets for certain messaging (to add value to each campaign) etc... This seems like a fun product to market.

As an agency founder and Creative Director, I am barraged by a ton of cold calls and emails from publishers and ad platforms trying peddle their wears--so that can be a long road, with slow return. You have an interesting and novel business model--does it have longevity? Well, that depends on your hustle. I would recommend the following:
1. Identify businesses would be low hanging fruit and are hungers to grow themselves and campaign to them directly (caviar, Grubhub, Eat24, Seamless etc).
2. Call up the agency admins and offer to send lunch (Chinese) with some collateral.
3. Use your own fortune cookie to advertise yourself (not only in your restaurants, but sending out the cookies themselves to potential targets.

I would start by calling media buying agenices. You're in NY, so no shortage of those :)

Sounds like you have experience cold calling, so maybe do the calling yourself until you gain traction with a couple of agenices. If you build good relationships and show results, you should get the same agencies coming back and buying for different clients.

13 yr business growth specialist here and mentor at ATL Tech Village. I think I need a bit more information before rendering any advice. This is what I understand so far - you have an idea, a website and some cold-call originated sales. Were any of these sales significant? Did they renew? When you said, "we have channels" did that mean, "we have exclusive agreements with these companies" or that you know who they are? When you originated the idea and began talking about it, who in the market space (other than yourselves) got genuinely excited about it? For anyone to suggest an answer to your question (cold calling or otherwise) would be a guess at best or irresponsible at worse (cash burning) until we know exactly who would "care" about your offering - your solution. Once we know this we can then decide how best to approach a marketing and selling strategy.

To handle growing your business in the advertising sector you must understand how advertising helps a business:
1. Increase Foot Traffic
2. Generate Leads
3. Introduce New Products
4. Highlight Product Enhancements
5. Get Traffic to a Web Page
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath