My company specializes in user and customer acquisition for mobile and web based products.
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.