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Not sure if you can find just trim piece or will have to replace the entire panel. Do a search on eBay for "98 00 lexus 75071-30060" that will give you a start. If you haven't already register at LexusDrivers.com and add the VIN for yours...that will give you at least the service that was done at the dealership.
The stainless steel trim piece isn't sold by itself from Lexus.
75075-30061 and 75605-30190 are the part numbers for the rear door and rear quarter panel trim assemblies. C0 at the end for black. Pretty expensive from the dealership, try finding a part-out in your local classifieds or on Facebook.
You can use "chrome" adhesive trim from an auto parts store. That's basically what the front and rear strips are from the factory, except custom fit. The sides are steel, but if you don't have the part, the adhesive trim will fill the gap. It just won't last as long.