Are you looking for the cat back portion? Thats the part that connects to the back of the down pipe and continues back to the tailpipe.
If not, the other type would be an axle-back, which connects (you guessed it) just in front of the axle and continues to the tailpipe.
I've actually been searching for a Borla Hush axle back as well. So far I found nothing though, so my $.02 would be to find a Hush cat back someone doesn't want and cut it off/ unbolt it where you need to to get it to fit if you need the axle back section. Or you could just trade someone for your existing cat-back.