If you're strong at IRL networking, you might want to talk to friends (and friends of friends) who are graphic designers. This has been a freelancer's market for design jobs over the past couple years as corporations have trimmed their marketing budgets but are still needing work done. A lot of designers recognize when the client's copy could use work, and can suggest a couple hours of copy rework at your freelance rate before they flow it into design. I have received work requests this way. It requires an ability to work closely with designers, as well as handling relationships with corporates.
A lot of writing for Web design going on this way, as well as marketing collateral. I've seen less in the way of print ads, but that's a longterm downward trend. Got friends in design? Talk to them and ask about their clients' copy. While $500 for a job is a nice sum to get as a freelancer, depending on the job, the client may hardly bat an eye, and it adds overall value and supports their brand.