are there any other big sites that i can buy unique articles from apart from DP what about helium and places like that
There are two threads in which freelance markets are discussed. If writers can find projects there means that you get articles from there too http://forums.digitalpoint.com/showthread.php?t=863978 http://forums.digitalpoint.com/showthread.php?t=854670
Another place to go is Constant-Content... they have pre-written articles or you can request articles to be written for the subject you need.
You can go to ArticleTerritory.com but it is in the 1st stages and so there are not many writers signed up, but soon it will be packed with unique content articles.
Well, Helium would only be unique if you placed an ad in the marketplace section otherwise it gets added in to their library to be voted on. You could request content on bidding sites like ifreelance, getafreelancer or scriptlance. You will get a bunch of really low bids there so before you take one ask for samples. It is important to judge quality before choosing a bid.