- since im not able to find them in the manuals, can anyone tell me where the Derate schedules are?

You would probably need an airport analysis for proper information. Software like Topcat has a database with most of the world's fields with all the declared distances for such calculations. It also has accurate performance for the 738. It lacks a bit of control if you know what derating, compensating and flexible really involve, but practically speaking... it's not bad at all.