I've actually done this exact thing to grow my personal training business from a one-man operation, to now, 14 trainers working for me.
I MUCH prefer doing free speaking engagements at corporations, for one main reason: bureaucracy.
As soon as you start asking for money from a corporation, even a small amount, you've now involved multiple decision makers, and your success rates will plummet. In my experience, with free speaking engagements, success rates (at least for me) are already 1 in 50. Imagine asking for money. If you do a free speaking engagement, it's really just 1-2 decision makers (some folks in HR).
When you're first starting out, I wouldn't focus on any particular industry. I'd do a "shotgun approach", and talk at as many industries as possible.
After each speaking engagement, track your stats - how many people in the audience, and of those, how many signed up for the retreat. Over time, patterns will start to emerge, and you can narrow your focus to just the most profitable industries.
The way I'd start getting speaking engagements is by asking people you already know who have corporate jobs. Look through your phone contacts, your Facebook profile, your LinkedIn profile, etc., and see who works in a corporation. Call them (it's better than email), explain what you're trying to do, and ask them if they'd be comfortable introducing you to either HR or someone in a management position. Don't expect 100% of people to say "yes" though.
After they agree to make the intro, about 50-80% of people will forget, so just gently remind them after a few days or a week.
Once the intro is made, talk to the decision maker, and explain what's in it for them. How do they benefit by bringing you in for a speaking engagement.
There's definitely way more to this, but this is where I would start.
