Would a salvage yard be a good option for you? Most P5s I see in my local yards have the original roof rack, but I think later models (2003.5 or Canadian models or something?) may have come with the filler strips. Even if they didn't come that way from the factory, you may find a car that had the rack removed and strips installed by the owner before it was put out to pasture. The good news is that I think those strips are metal, and wouldn't have fallen apart in the sun like the plastic parts. You can set up an alert on row52.com to notify you of any new P5s. If you get lucky, maybe someone will have already removed the headliner for their own needs.
Other than that, you can look on car-part.com or any other salvage site from time to time and look for cars that have pics to confirm whether they have the strips or the rack installed.