Many larger webmaster and writing forums have sections where people offer their services for hire. You'll find plenty of offers from authors willing to write articles for a fee. Here area few things to keep in mind though:
1 - Don't expect quality, well-written and unique articles to come cheap.
2 - Make sure the "author" doesn't simply lift someone else's work from the web and re-write it (or not). Getting hit with a DMCA notice is no fun, especially when you paid good money for the article in the first place.
3 - Make sure the author knows what he/she is talking about. In other words, you have to know enough about the topic to be able to determine if the article is factually accurate and truly useful to your site's visitors. If it isn't, your website's reputation will suffer. I hope this helps.