The National association of science writers has many resources for members. I don't know about membership costs, but it's a good organization.
Trade pubs and hospital magazines are a good market to look into as I know most still use freelancers. I'm a lifestyle editor for a pub company and see good clips from these types of markets.
Some of the juvenile magazines need well-written science articles geared towards younger audiences and accept freelance subs and articles.
As always, read the big names like Scientific American, Wired, Science, etc to stay up to date on what's being published and what is happening.