Whenever I've done this type of stuff, I've just farmed out my article writing to elance.
Some article writing services do pretty much the same thing, being essentially middlemen rather than having talent inhouse, which is why I don't recommend them, but YMMV
When I found three writers I trialled, it was through elance. Tracked down their portfolios and contacted them off-site. One was in Australia and I believe they outsourced the writing to Asia and the quality (writers who weren't native English speakers) wasn't great. The one I favoured was from the UK. If she was outsourcing, it was to decent writers.
Some article writing services do pretty much the same thing, being essentially middlemen rather than having talent inhouse, which is why I don't recommend them, but YMMV