What are the best platforms to find or advertise an app development service on partnership basis?

If I understood correctly, you are planning to create the app for free in exchange for percentages/shares in the business/startup?
If so, the best option would be to partner with accelerators/incubators and other organizations that work with startups who have good ideas. I personally work with hundreds of startup each year, so if you're interested, we can talk and possibly collaborate.
Good luck

Difficult to answer this question, as the answer relates directly to type of App.

Might be useful for you to book calls with various people off Clarity, where you describe your App + Business Model, then describe a unique marketing flow for each App.

Hi there,

Having worked in the technology space for over 6 years and established strategic partnerships, there are several ways you can look for firms or individuals that needs your services.

From what it sounds like, you are looking to help people outsource their app development/support for their company While there is definitely a market for this type of service, you need to start getting granular and drilling down to the ideal industry that needs your services. You will need to employ a dual approach of cold outreach and strategic marketing to generate your business partnership.

First, you need to narrow down the ideal types of businesses you want to target. What types of companies or individuals are in need of your services the most? Why? What is the value proposition for them to use your service and why should they care?

After determining this, it's time to put together some marketing campaigns to help drive the value proposition of your service. There might not be a best platform to find or advertise your service- you will need to try many different avenues. Perhaps you can start here on Clarity.

I've spoken with hundreds of companies in your shoes so I am happy to jump on a call and communicate several methods that might work for your company as well.

Best,
Kevin

Techopedia defines a mobile ad platform as an «intermediary between mobile publishers and advertisers.” In layman’s terms, a mobile ad platform is a place where advertisers can purchase ad spots that are delivered on mobile websites and apps. These platforms work two-ways. With the basics out of the way, let us look at some of the best ad platforms for mobile advertising. With Google ads, ads can show on both desktop and mobile devices.
Instagram is another good option for advertisers since it’s a mobile-first platform created with the mobile audience in mind. While Bing may have smaller market share than Google, your advertising budget could spread farther on the 2nd place search engine. On average, you will pay 70% less for Bing Ads than Google. Lower costs are not the only advantage of using their Bing's mobile ad platform, though. Since being acquired by Google in November 2009, Ad Mob has become one of the most popular mobile ad platforms. The platform is beneficial for companies aiming to monetize by showing ads in their mobile app.
Tube Mogul is a demand-side, mobile ad serving platform that started the non-human traffic credit program in 2016, concerned that their clients might be flooded with non-human traffic. Without having a clear idea of who sees your ad and which ads perform best, mobile ads are a waste of time and money. Key metrics include CPC or CPM, the total number of conversions, and CTR.
But it is a good start to look at the statistics for each solution for a mobile ad platform comparison.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath