I might have an N/A downpipe laying around - I'll check. There is no OEM flexpipe, there is a union flange between the cat & downpipe that uses a ball & socket type gasket to allow flex. I've used an aftermarket flex pipe with every one of the exhaust systems I've welded up.
Just be wary of the parts you've found - most of them are probably univeral and not "direct fit" as they might try to lead you to believe.
IMO - I would bring it to an exhaust shop & have it quoted, they'll do the whole thing in larger diameter pipe that will perform better, much cheaper than trying to piece together an OEM system, IMO.