Where do you go for alpha users?

Ideally find some place where people are complaining about the problem you solve. If your product is super early stage, reaching out to 5-10 targeted people is usually better than getting a couple hundred views somewhere like PH. Depending on your product you may have luck searching for key phrases or names of competitors on Twitter, Reddit, and FB groups.

I've had some decent luck with Reddit. There's a lot of subreddits that allow self promotion. Smaller sub reddits that you participate in and are related to your product can also help. Larger sub reddits have strict self promotion guidelines, but you can still post there if you're careful about making the content of the post meaningful.

You need to do the following.

1. Define your target audience.
2. Find places online where your target audience is spending their time.
3. Once you've zeroed down on targetted platforms, you need to engage and create value there.
4. Once you've a decent reputation, you can post about your app and request beta/alpha testers.
5. Provide them with some kind of reward for testing your app and offering quality feedback.

Example:

Your App: Tinder

1. College going and young adults
2. Instagram, Snapchat, etc
3. Create viral content and comment on the content created by your target audience.
4. Create a post telling them that you've a new dating app and need people to test your app.
5. Rewards can be some kind of vouchers or giving them 3 months of your paid package, etc.