Found these exhaust tips for the 335i, but Im sure they could be installed on the 135i.
(
If Maddad had these on their whisper exhaust, Id already have it on my car. The tips that come on the whisper look too much like the stock tips from the 128i.)
Says the price is $99 for the pair. They come in Powder coat black too.
http://www.dimisatips.com/buy.php