Just wondering if the headers and flex pipe off a 2000 ex sedan will fit my 2000 ex coupe. Both are 3.0. Reason being is I have a aftermarket dual catback exhaust I want to install in the coupe but the stock dual exhaust has the cat welded to the flex pipe so I'll have to replace it from where it connects to the headers and I found a new set of headers and flex pipe assembly from a friend of mine but it was purchased for the sedan.

 

There's a number of places that sell parts on-line, and use Honda part numbers in their on-line catalogs.
Rock Auto, Majestic Auto, for example and there's a few others.

Look at coupe and sedan & check if the part numbers are the same.