Bookstores like Borders and Barnes and Noble offer extensive newsstands featuring magazines you won't find in your local grocery store. You may also find publications that aren't listed in Writer's Market. Check Writer's Market for free back issues. When ordering a back issue, save yourself a SASE and ask for the writer's guidelines at the same time. Some GLs are excellent, giving you far more information than you can glean from the Writer's Market entry.
Many publications post extensive archives of articles on their websites.
Print a few articles and read them carefully to get an idea of their tone, style, content, and depth of coverage. Even if a magazine doesn't publish articles online, it may offer an index of back issues. This will give you an opportunity to determine what has already been published in the past year or two, so that you don't waste time with an article or query that duplicates past effort. Look for guidelines online. Many magazines are now posting their GLs on their websites.