I usually use the flyaow links page and use the Carlson Wagonlit ITN interface. It's the same engine that UA uses.

Do a dummy booking, and before it wants a credit card you can hit the "select seats" link.

Just bear a few things in mind when looking at seat maps:

1) There may be less inventory than seats that show available, as some people may not be assigned seats until checkin
2) Seat maps can and do change often as people change flights, seats, or get upgraded

I've found the ITN seat maps to be generally pretty accurate though.