If I was in the market for a new sofa, I'd probably look for one like this...
...an amply proportioned sofa with a deep seat, rolled arm, and tight scalloped back. Doesn't it look comfy?
As I mentioned in my Dream Home post, Sofia Coppola's blue one (top two images) is the Jules sofa by George Smith (as is probably some of the other sofas pictured as well). Unfortunately it's quite pricey (I believe around $12K). The Cameron sofa from Room and Board (which can be upholstered in any fabric they have available) is a good option for getting the look for less. I'd probably replace the legs with vintagey ones to get closer to the look. If you happen to know of any other sources for getting a similar style piece at an affordable price please do share!
UPDATE - From my awesome readers:
Meghan Carter - The Miramar Sofa from Serena and Lilly
Beth Chodak - The English Roll Sofa from Restoration Hardware