Check out
www.jamesallen.com
Was fantastic when i bought my engagement ring last year. They're made in the USA so no duty is charged. Plus you actually get to select the diamond you put in the ring. In other words each diamond you can see online so you can view where any defects in the ring are, so if its a high quality cut but has some defects that would be hidden by how it's mounted in the ring you can get it for cheap.
I ended up spending around 2500, however in store for the same ring it would have been over 4000.