To see the number of seats left in each booking class (non-redemption booking), ITN/Amex Travel website will do fine. However, in some cases it is not necessarily 100% accurate. I was booking a ticket for my brother CGK-SIN-LAX on a particular date through my travel agent. The SIN-LAX SQ20 leg was shown as something like Z4 C4 J2 D1, also bookable from SIA website.
My bro was put on waiting list (booking class: Z, which is supposed to be the highest of business class -?-). I guessed that this may have been some complex revenue management system at work. He is now confirmed though.

To see the seat maps and to see the seats taken (reserved) on any particular flights in F & J (NOT to see how many seats left as there are some passengers who booked a ticket but have not reserved their seat numbers), make a dummy booking on the SQ website, and it will come to a screen where it says "Select your seats" - thanks to a fellow FTer who posted this method a while ago.