You are quite right! There are a lot of sites which provide article writing. Search in Google. I personally use one company from India and because I have a lot of clients they provide relatively cheap services. $5 per article - 500+ words. I'm checking them every time on CopyScape - http://copyscape.com and they are always unique! :-) So search for such kind of companies - in my opinion they are the highest quality ones!
I love https://www.freelancer.com/ - a bit pricy, but the writers are from USA and UK - so they are very experienced and native speakers!
... and of course the largest site for people outsourcing their services including high quality article writing on every topic - https://www.odesk.com :-)
100% - https://www.elance.com
There is one site which is specialized for article marketing and article writing. It is called - https://www.iwriter.com/ and I would suggest using it. Make sure to choose a native writer and a content 750+ words - only this works nowadays - no bold, italic, keyword density - all this is a history! Make sure to check all your written articles in copyscape!
Here is one site for finding professional article writers called https://www.textbroker.com it is pretty easy to find reliable writer there! use it with confidence and ease! Hope that helps! BTW - great suggestions I am seeing here! thank you all