Need to buy a replacement usb adaptor for my apple pencil.

Apple does not sell these directly.

That adapter is part of the entire Apple Pencil package.

I suggest contacting/reaching out to Apple Product support and services.

Tap on the first link below and scroll down some to get to the Apple Product Support section and phone numbers for your location/region.

https://www.apple.com/contact/

OR

https://support.apple.com/en-us/HT201232

Best of Luck to You!

Plenty on eBay at prices starting at £3 including postage. I bought my wife a couple of spares just in case

An example

https://www.ebay.co.uk/itm/274333748768