Truth is there are lots of companies offering article writing and even submission service which makes it hard to spot a lemon from their huge numbers. If you are trying to find a service that offers great quality articles at very affordable price, first check out their reputation then atest their service. I always do this when searching for companies online.
There as many writers as there are grains of sand on the beach... and nearly as many sites offering... Determine type of articles/subjects then go where they exist, see who are the writers/authors of note, then contact them. But there's little need to build a directory/list of these guys... Bing and Google do a pretty fair job of that already. :)
If you use the services that everybody else is using, won't you end up with the content that everyone else is using?
Maybe there is a way to tap into the expertise of people who have experience with a subject without having to shell out large amounts of money?
As a publisher, YOUR value comes from your ability to find such experts and get them to contribute. If you are able to obtain it with very little work, well, wouldn't your competitors just do the same thing?