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 121762 SHELF SUPPORT BRACKETS PINS FIXINGS PEGS BRAND NEW SET OF 4 | eBay
Spare Hardware Parts Billy Shelf Pins (Compatible with IKEA Part #131372) (Pack of 12) : Amazon.co.uk: DIY & Tools
IKEA BESTA Shelf Support Pins Plugs Fixings Brackets X 4 GENUINE SPARES 113301 | eBay
GRANHULT nickel-plated, Bracket, 20x12 cm - IKEA
Concealed Shelf Support, For Use With Plug In Supports, For Wooden Shelves
Replacement for IKEA HEMNES Shelf Pins 104171 (12 Pack) : Amazon.co.uk: Home & Kitchen
Ikea Shelf Brackets - Shop online and save up to 17% | UK | LionsHome
Spare Hardware Parts BESTA Shelf Pins (Compatible with IKEA Part #113301) (Pack of 12) : Amazon.co.uk: DIY & Tools