Spare Hardware Parts Billy Shelf Pins (Compatible with IKEA Part #131372) (Pack of 12) : Amazon.co.uk: DIY & Tools
IKEA Billy bookcase tip: if you have an OLD Billy bookcase (pre-2014) you can still add extra shelves even though the new extra shelves from IKEA use a new, disc-shaped shelf pin.
The Best Way to Fill Billy Bookcase Holes
Spare Hardware Parts Old Billy Bookcase Pins (Compatible with IKEA Part #121762) (Pack of 12) : Amazon.co.uk: Home & Kitchen
3mm Shelf Studs Supports for 3mmØ Holes Choose type Quantity IKEA Billy Bookcase | eBay
IKEA BILLY Shelf Pins #101532 – FurnitureParts.com