How to buy the end cap for the 1st Generation of Apple pen

Lots of these found on Amazon.

Best of Luck to You!

I am taking a different approach. When you say end cap, do you mean the cap on the bottom of the pencil that covers the lightning connector?

Something like this?

https://www.amazon.com/Replacement-Magnetic-Compatible-Protector-Convertor/dp/B07VGRMYQ6/ref=asc_df_B07VGRMYQ6?tag=bngsmtphsnus-20&linkCode=df0&hvadid=80882941400123&hvnetw=s&hvqmt=e&hvbmt=be&hvdev=c&hvlocint=&hvlocphy=&hvtargid=pla-4584482468084745&psc=1

The online Apple Store also sells them:

Apple Pencil Tips - 4 pack - Apple