You can try www.itn.net. Registration is free and enables you to use "expert" mode which allows you to see seat availability by fare class.

To see a seat map, do a pretend booking on the flight(s) of interest, and you will see both a fare calculation as well as a "Select seats" button which will bring up the seat map. Just cancel the itinerary when you've obtained the desired info.

Cheers,
FewMiles..

[edited to fix link]

[This message has been edited by FewMiles (edited 09-24-2000).]