I did not find a seal available separately.

You could try flipping the seal over so the new surface seals better. For bonus points, some Permatex UltraBlack on the back side (formerly the front side, before th flip) to fill in any cracks might help too.

I've done that sort of repair on many seals when it is not sold individually, or when I need an immediate fix.

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2001 X5 3.0i, 203k miles, AT, owned since 2014