Your best bet would be to build your own like the previous 2 suggestions before this post
However, if you don't feel as if you are capable (it is easy enough but I know it can be nerve-wrecking), then you could try the local PC store to see if they would do it for a cheap price.
That way, you get the best components for the best price. Add in the satisfaction of making it and telling people you built it, and it is the best way to go.