Where do I go to find out what a part number is for something?

I am looking to replace the drivers side window seal and I don't know what the part number would be.

Thanks

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John

1995 E320 - 115kmi+ Smoke Silver/Cream (Wife's Wagon)
1989 260E - 195kmi+ Black/Grey (My Panther)
1984 300D - 242kmi+ Black/Palomino (Retired)