I don't have a great answer if you have a limited budget and time, but I would focus on

1) Building a social media strategy with an incentive plan attached to spark engagement.

2) Create a "hit list" of companies that fit the profile and ask them to participate. Not sure how many companies you'd need for the beta but this select list could have special privileges and guarantees such as free or reduced cost when it launches.

3) Check out launchrock.com. For $5 per month you can build a acquisition website and get free promotion to their audience which I think is 200K+ people. Not sure the demo of those people though.

4) Rent a list of contacts in your target market, perhaps through a trade publication that caters to your audience, and send an email (or series of emails) asking for their participation and telling them why they should participate and what they will get for it.