How to find developers for a startup with next separation of the business?

I'm a software developer and I have made the first alpha version of my service. And now I need more human resources to make something significant.

I need people which will work for free yet. And then we'll make a commercial versions (or somehow make money), so, everybody will have his his portion corresponding to some agreement.

By this agreement the portion of each developer is defined by his contribution to the project.

Is it a common practice to find collaborators with separation of the business?

I am not sure how much is down to language but you need to be able to sell a dream and a vision to prospective partners and wording such as "or somehow make money" will not do that.

As a developer put yourself in their shoes, what what convince you to want to work on something for nothing with the chance of maybe making something at some distant point in the future?

You need a business plan which shows exactly what you are going to do at every stage and how you are going to get there with timescales and, most importantly, what return you are going to give them when you get there.

You need to portray and ooze belief and passion for your product/service and be able to infect others with that same passion - if you cannot then you are going to struggle.

It's a commonly-sought practice but terribly hard to pull off. Developers who are creative tend to already have their own personal projects and you'll need to earn their trust and win their aspiration to get them to work for you for a piece of the business. Remember that your business isn't making money yet and thus it's price is zero. 50% of zero is zero, so you'll really need to strive hard to win them over.