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.
IKEA BILLY Shelf Pins #101532 – FurnitureParts.com
Milsbo Extended Shelf Brackets, for Ikea Greenhouse - Set of 10 - 3D p – hobbypilled
GRANHULT Bracket, nickel plated, 11 ¾x4 ¾" - IKEA
Ikea Shelf clips Fabrikor glass door cabinet, Part # 114594 (2 pack) - NEW | eBay
Amazon.com: Spare Hardware Parts BESTA Bookcase Shelf Pins (Replacement for IKEA Part #113301) (Pack of 12) : Home & Kitchen
Ikea Shelf support pins, Part # 128643 (4 pack) - NEW | eBay