How do I get another cap for my Apple pencil?

maybe

Amazon.com: Hiicopa Magnetic Replacement Cap for Apple Pencil 1st Generation - with Silicone Protective Cap Holder (White) : Cell Phones & Accessories

Amazon.com : apple pencil cap

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another cap for my Apple pencil - Google-

While Apple do not officially sell replacement Caps for the first generation Apple Pencil, there are many third-party replacements available from Amazon and eBay.

If you visit the Genius Bar at your local Apple Store - and ask the technicians very, very nicely - they may be also able to help.

Genius Bar Reservation and Apple Support Options - Apple