ForumsAdvertising, Sales and Marketing

You are correct in that nearly every industry and business size (from SMB to enterprise level) requires the work of a graphic designer. The problem is that mid-size to enterprise level companies usually employ their own "in-house" designers. So the biggest challenge in marketing your business (and yourself) will be to differentiate your services from what those companies are currently capable of. And the best way to do so is with a phenomenal portfolio and client list. Not knowing anything about your business, I can only assume that you're in the process of building both.

If you're looking for particular industries to target, again you can see success with nearly all categories out there. But from my own personal experience, here are some industries that I've found to be heavily dependent on graphic design services...

Advertising
Ad agencies and marketing firms require a ton of graphic design work. Not only is the demand there, but the work you will do for most agencies will be "high profile" and look great on your portfolio. Yes, agencies usually have their own graphic designers. But don't let this deter you from soliciting them. The reality is that most ad agencies outsource a great deal of their design work and use freelancers. This is usually due to sheer workload on their in-house designers, or because client needs fall outside of their current capacity. Moreover, smaller boutique agencies may not even have a full time graphic designer. These are just open invitations for you to establish both short and long term contracts. I've worked for some of the biggest names in advertising and can tell you that EVERY agency I worked for outsourced graphic design work at one point or another.

Entertainment
Like advertising, the entertainment industry is a very visual one, and requires constant design work in almost all forms of media from print to web design. I know some graphic designers that have seen success by targeting the film studios and TV networks. Don't forget to solicit the intermediary companies (i.e. print shops, marketing, PR) who are already partnered with the studios and networks.

Publications
Fairly obvious one here. Consumer pubs, trade magazines, newspapers etc.

Architecture & Real Estate
These businesses utilize graphic design regularly for print and web. I have a friend whose startup graphic design company just landed a large and lucrative contract with a commercial real estate developer. He now does all of the computer renderings for their property developments.

Anyways, I know I'm just scratching the surface, but if you haven't thought of these industries then hopefully it might get the ball rolling.

Good luck!