If you're looking for a 128i M Performance Exhaust now, you can pretty much give up hope. There may be one squirreled away at a dealership somewhere, but there are none left in any BMW warehouses and apparently none in Germany either. I tried for a month to get one with no luck.
Fortunately, you can use the 135i PE with a little work. The 135i PE is a little more expensive since it includes piping, but it's your only option at this point. The 128i PE comes pre-cut with about 2" of pipe out of the silencer body. You will need to cut the 135i PE in roughly the same way.
BE WARNED! I was super-careful to measure away from the silencer body with calipers and I cut the pipe in the same place that I cut my stock 128i exhaust and they were still about 1cm off. I suspect the silencer body is very slightly different or maybe it's a minor tolerance issue.
You have two options to attach the 135i PE. The inlet pipe on the 135i PE is a hair under 70mm. The inlet pipe on the 128i exhaust is around 66mm so it's not a perfect match. A good muffler shop should still be able to weld them even with the difference in diameter. If you weld them though, it's obviously a permanent attachment.
I took a slightly different approach. I cut a small section of pipe about 1.5" wide directly below where I cut the silencer off. I mounted this over the existing stock pipe using JB Cold Weld to create two pieces with the same diameter. Time will tell if the JB weld will hold. Since it's not really structural, but just a "spacer," I think it will be okay. If not, I will have a muffler shop weld the spacer onto the pipe. The reason I did this is so I can use a sleeve over the butt joint. This way I can remove the PE easily someday if I want to sell it.
The sleeve wasn't hard to find since 70mm is equal to 2.75" which is a common diameter. I used a Vibrant Performance #1176. It's extremely thick and stout. The basic ones they had at my auto parts store weren't going to cut it. All mounting points on the 135i PE are identical so installation is easy. The exhaust tips on the 135i PE are slightly larger I think, but it still fits the stock opening on the 128i just fine. I'll take a few pictures of my installation later and I'll make a sound clip when I get a chance.
