You need to publish tidbits of how to accomplish this online for free to become an authority on the subject. When you become known for this then companies will pay you to consult on it.
In the short term, partner with development shops who build apps for customers. Get them onboarded as resellers and have them recommend you for the post launch acquisition of customers.
Check the store on http://chanimal.com for a great reseller program kit to get started with building a reseller program. Call me on here if you want some more help.
It might be a good idea to start from start up meetups, startup social / professional networks and linked in. As startups tend to clearly state the their early stage status on most of their profiles, it would be quite easy to target start up companies by targeted ads both by using date of establishment as well as startup related keywords to narrow down the audience.
It's important to note that:
1) Early-stages startups are not interested in such services because: they don't have money for it / it's too early to scale anything until traction.
2) The other thing - more mature startups: seed, Series A and later.
It's right audience. Because as soon as you prove your hypothesis and get a traction, it's time to scale it.
But in this case it's better to formulate a question:
"What kind of marketing channels I need to use?". It's a very wide and abstract question.
But usually it works like:
1) Direct Sales (confs, linkedin);
2) Content Marketing (Good guestposts);
3) Partners program.
In reality there is no big problem. Any startup which crossed "traction line" has a need to leverage more quality traffic. And I don't think that it's a problem to reach them.
The main problem to provide them with really good user acquisition in terms of Price / Quality. It's a hardest part.
There are thousands companies who do the same. In my taste, only 1% provides service which worths of attention.
Use Angel List and Startups.co to create a database. You might even be able to write a little web scraper script that automates everything and populates a spreadsheet for you. Then, activate Yesware on your email accounts, and use it to split-test the open rates with different subject lines. Once you have 2-3 subject lines that get high open rates, you can begin move on to split-testing the email content. Once you have a soft pitch that's getting a decent response rate, you can turn those initial "sales" calls into customer development interviews. By asking the right questions, you can figure out exactly which pain points you should be using in your positioning. Then, hit the accelerator. Direct email and phones calls aren't sexy. But they are a traction channel until you find something else that you can automate.
Let me know if you want to discuss further.
Austin L. Church
The most obvious place to start is potentially Upwork and Angelist. A ton of companies need both paid/organic growth consulting. You can also hit up LinkedIn to find companies that potentially need agency/consultant help.
The one caveat is a lot of these companies are either bootstrapped and or just got an initial round of funding which I am assuming is being spent on engineering resources and potentially some initial growth to get some traction.
A couple tips to get some paid growth gigs for your agency:
Upwork, CloudPeeps, Angelist, and Growth Geeks are places to start.
You need to be aggressive and hound for paid/organic growth work. There are a lot of people who do this as well as tons of agencies. You also have to understand what type of pay you will get since as you are an agency vs. a individual consultant. (Unless your a 1 man agency.)
Lastly networking is a great way to meet companies. Go to happy hours/meetups/conferences and meet people on the growth teams at more established startups/companies if you can. Meet them and potentially pitch to offer services at a latter date. Put together compelling reasons why your added % take in addition to actual paid user acquisition is worth it to their overall growth strategy.
Finally, understand that there is also an increasing shift to bring mobile UA in house, so thats definitely something to consider.