Here are some things I would try.

1. LinkedIn Advertising.

Why?

Because it seems like your app is designed for a very specific type of person and on LinkedIn you can target users with ads by job description (amongst other things).

In short, if you've got a pretty good idea of the job titles, skills, industries your target audience is in, LinkedIn might be a good option.

You could send users to a company page (where you register interest in a product).

Build your network (by building up your audience on LinkedIn).

Or send users to a dedicated landing page where they can download the app/register their interest.

2. LinkedIn Group Discussions

- Find and sign up for groups on LinkedIn relevant to the app you're considering
- Propose the solution you're considering testing
- Ask people if this sounds like something they would be interested in
- Ask for beta testers to sign up (by emailing you or to a mailing list)

3. Facebook Ads.

This approach would be similar to the LinkedIn Advertising. Set up a simple landing page (using something like Unbounce or Leadpages) or whatever you have access to, create some niche audiences using Facebook ads and see what happens.

4. Podcast shout out.

Look on iTunes for a podcast related to your audience/industry.

Set up your landing page.

Pay the podcast a small fee for advertising/referencing your service and give listeners an exclusive discount.

5. Twitter ads.

Same as Facebook/LinkedIn but with Twitter.

You might want to use the "lead gen" option to collect email addresses and then go from there.

If there are common keywords that people in this industry are likely to put in their bios or reference in their timelines, you can use a tool like Followerwonk to build a niche targeted list of 100 users to target with advertising on Twitter.

6. Reddit could work.

You'd follow a similar process to that noted above but instead of finding users on Facebook, Twitter, LinkedIn, you'd have to spend some time finding relevant SubReddits to post in.

7. Google Ads.

If there are specific, niche keywords people use in this industry you could set up a landing page and then send traffic via AdWords.

The key would be making sure your keyword research is super tight. You might just want to use exact match keywords for a campaign of this nature.

8. If you have them already, message your LinkedIn connections.

Ask them if they would be interested in the product you are offering and if not, would they have any recommendations for people who might be interested.

You can connect with the people they recommend on LinkedIn, or just ask for a bit of word of mouth support.

9. Meetup.com

If it helps to keep things small scale, maybe you could go local.

Use meetup.com to find groups that are relevant to your audience.

Attend their event. Demo your product/service and see if anyone will buy or is interested in testing. This is a really useful way to get direct feedback.

10. Target followers/users of any competitor products.

You can do this in most of the advertising platforms noted above in one way or another.

You can also do this on Twitter.

-Find a competitor account.
-Put their Twitter account name in the tool Followerwonk
- Download their follower details
- Go follow and engage with some of their followers
- See if you can build up a small audience who you already know are paying for something similar to what you've created

11. Another Twitter option.

- Download Tweetdeck
- Set up a column that filters by keywords related to your product,service,app or industry.

If anyone shows any indication they might be looking to buy a product like yours or maybe you can just help them in some way, start Tweeting them.

Costs you nothing but your time on this one.

Hope these ideas help! Good luck with your app!