Academic articles are usually found in academic journals and books, but they are not really necessary to get a good mark in the TOEFL. What you need to do is develop all of the four skills tested by the TOEFL: reading, writing, listening and speaking. This requires that you enlarge both your active and passive vocabulary AND that you get used to the format of the test (I personally found the speaking part daunting ). (You need a good grasp of grammar, too, but that was not your question, right?)

Read as much as you can. Magazines such as the Economist, the National Geographic and Times are very useful and available online. This way you'll kill two birds with a stone. First, you'll come across some useful (and sometimes sophisticated) vocabulary in context -- which comes very handy, as TOEFL tasks can be pretty much about anything, from biology to psychology, from economics to archaeology, so the more the better. Second, you'll also get some ideas for the "free" piece of writing -- I mean, the one which doesn't draw on the comparison between an article and an extract from a lecture.

Anyway, back to your question, if you still want to read academic articles, Google Scholar is a good tool to start your search (just put some keywords in the box). The vast majority of the articles you can access through the web require either a payment or a subscription (libraries and universities are generally subscribed to a number of electornic journals), but you should also be able to access some free resources. Good luck!