Where to promote my app for affiliate & internet marketers?

I would recommend you to promote it in social media platforms. Mostly the targeted audience resides in the social media platforms now a days, and I don't think there would be any better place to promote the apps other than this.

What do you guys think of solo ads?
Would this product be a right product to promote through solo ads?

It sounds like you're a developer. Great! That means you have a marketing channel available to you that few others have, which is a great advantage to have. That channel is Gabriel Weinberg calls "Engineering as Marketing" in his latest book "Traction".

Build free tools that would be useful to people who might also find use for your paid product. Think about Moz and its Open Site Explorer, HubSpot and its marketing grader, or SEO Book's PPC test calculators. The reason they build these tools is to create something that will be used and shared by member of their target market. The traffic it attracts will be potential customers for their paid services; they're opportunities to advertise those services and collect e-mails to warm up prospective customers.

If you don't have the engineering talent but can write, you can do the same with content marketing. Build mini-sites, e-mail courses or whitepapers. Give them away to attract traffic on social media and search. Use the leads to sell your real product.

There's always more traditional advertising channels as well -- PPC, display, social ads, etc. I wouldn't spend much money on types of advertising that don't exist outside the "we call ourselves internet marketers" space, like "solo ads". You've never heard of Intuit buying "solo ads" from "WSOs" to sell QuickBooks to business owners.

If you want to reach more customers and faster, why not try to promote your APP on pay per click? That way, you will get more eye balls seeing your APP and help you get far more customers and faster.

You can target them by specific hashtags on Twitter, engage and acquire marketers from there. Automate it even.