Unfortunately I don't think there's a good way to find these companies unless you happen to live near them or work in the industry they sell to.

In my experience, they normally get started and funded by local people in their industry and they sell back to local customers (at least initially) from their network, so there's very little in the way of advertising, especially at a national level. Also, they often hire from their network for early positions and local colleges/universities as they grow, so you might not even see them on job boards.

If you're looking for companies that are a bit more established, you could try industry trade shows.