yes there are plenty of 2.5" ones on ebay, matter fact i just got my buddies new ssac header for 98 shipped. Yes i said it 98 shipped.
just type in sentra 2.0L header and it will come up. Just look at the size of the secondary where the flange is, If the pipe comes really close to the bolt holes its the 2.5" Just look for the cheapest one, It should come out to around 98 total. Look at the picture of the gasket, if the outer ring is really thin for the flange to cat gasket its the 2.5". Hell even the ones that advertise it as 2.25 but when i looked at the pic its obviously the 2.5 and it measures out to it. Even the guy selling them that is holding the tape up all crooked saying its a 2.25 if you look at it closely in the center more comes out to right at 2.5"
Just look man, again type in sentra 2.0L header and it will come up, guaranteed. No one advertises them as 2.5" You just have to trust it by the looks of the pics.