Any 3" sr downpipe will work.
The standard down-pipe with the flex location at the bottom is alright if your car isn't horribly low.
A vast majority of down-pipes have a slightly narrow neck, but it isn't anything to worry about.
Right now my car has an XS Engineering down-pipe which gets rid of the cat and mounts directly to the catback.
Ground clearance is pretty good, and it gets rid of a flange, so one less thing to catch on the ground/leak.
http://www.frsport.com/XS-Engineerin...ET_p_9402.html