I've always liked the idea of commissioning articles but it's hard to make them profitable, especially if you need high-quality copy and you don't like plastering too many ads on the page (meaning less income per article).
I've just found a new source which looks promising and I've purchased an article about the digital camera RAW format for MediaCollege.com. I'm going to keep an eye on the stats and see how it goes, and I'll probably purchase a few more similar articles as well - enough to do some decent testing. If it goes okay this could be a great way to help build the website.
BTW I only ever purchase articles that are 100% unique.