Wokke.de. Wolf will sell you the lips to make custom 3-piece style 5s; they aren't the same as RS lips - the RC centers are slightly larger apparently. You'll just have to take your barrels to a machine shop to have the lips cut off. Seems pretty straightforward but I think he said he would send intructions with the purchase of the lips. Thinking about doing this myself for my E36 since I have a beat-up set of style 5s laying around. Don't know if it's worth the cost, but it would be pretty badass...
BTW, I've bought two sets of wheels from this guy in the past and he's well known over on BFC. Awesome guy to deal with and believe it or not, getting a set of rims home from Germany was not that expensive.