You can check out the Berk Technology Street Performance Downpipe Back Exhaust System for your 135i. We've installed it on one of our customer's 135i and it sounded great and I think this is exactly what your looking for.
[IMG]http://www.**********s.com/images/Products/Berk/berk-135i-exhaust-3.JPG[/IMG]
[IMG]http://www.**********s.com/images/Products/Berk/Downpipe-Back-Street-Performance-Exhaust-3.jpg[/IMG]
-Frank