If you do a search for flights on Orbitz, each flight has a line that lists class of service/flight duration/aircraft/ and a link "view seats" if its available for that airline. This will only show assigned seats, there may be more "sold" seats that have yet to be placed so its not always accurate.

Expedia has something similar that is currently working for US thats something like "preview seating" but doesn't show the layout as well as Orbitz (some Envoy bulkheads not shown).