NEW Shelf Brackets for IKEA DETOLF Add 3 Extra Shelves - Etsy
GRANHULT Bracket, nickel plated, 11 ¾x4 ¾" - IKEA
Amazon.com: Spare Hardware Parts BESTA Bookcase Shelf Pins (Replacement for IKEA Part #113301) (Pack of 12) : Home & Kitchen
Replacement Shelf Support Pins / Pegs for IKEA Part 101532 (Pack of 5) | eBay
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.