A good graphical representation will enhance the given topic/service and/or product's relevancy, a significant factor in conversion. With that said, the need for your service(s) is not limited to a certain 'type' of business. In previous years, it was difficult to track effectiveness of graphical presentation(s), but with various analytic programs available nowadays, it's easier to obtain the conversion data of graphical representation(s).
Having a tracking-record of your work will make your portfolio more attractive, but it's not a necessity as many companies evaluate the quality of your service by its visual aspect(s). As pixeltorch stated, taking a proactive approach is worth the effort. If not found, make yourself to be found
From looking at your portfolio, I noticed some Halloween related product designs. Perhaps, you could contact companies like OTC (Oriental Trading Company), FrightCatalog, etc. They probably do have in-house staff working on their graphics, but during their busy seasons (Halloween and Christmas), there might be some opportunities for freelance work.
Best wishes to you