in business (or small businesses) what opportunities for writing exist?

Small to medium advertising agencies use freelancers all the time, especially if you are a subject matter expert.

The same goes for medium to large businesses that have in-house marketing departments. They can get overloaded, for example, during trade show season for their industry. Learn the cycles of the businesses you will be approaching.

The problem dealing with small businesses directly is that they typically lack marketing experience and they will make tactical decisions without a sound communications strategy.

Develop your own promotional mailer and make it really distinctive. Send to production managers in agencies (they keep the list of trusted resources) and marketing managers in corporate environments. Follow up with calls and a parallel email campaign.

Good luck. You can do it.

I write fundraising letters for small organizations that I have become a part of or affiliated with in some way. Also, you're on the right track with small businesses, remember, they also need press releases and other marketing documents - product descriptions, new venture announcements. Many foundations have monthly or quarterly magazines they provide and often need researchers and writers. That may be an option. Hope that helps.

Consider writing ebooks. Good information on this site.