Sorry to resurrect an old thread, but I was searching for the same thing and came across this post but no answer. I reached out directly to Bilstein for their confirmation.
That retainer/spacer is called the "Boot Adapter" by Bilstein and is part # E4-B36-398C.
Summit Racing does offer these adapters, but they can be hard to find in stock. If NOT using the boot but only needing the adapter, you might also try a 12mm "shaft collar" that are readily available. Roughly 12mm ID, 22mm OD, so plenty of width for your washer to sit on. 12mm is the diameter of the narrower upper section of the Bilstein shaft
You can find the boot itself (part # E4-SR1-Z012A06) but it does NOT come with the adapter.
I know I'm not helping OP, but for the next person who comes along with the same question, I thought the thread should have its answer!