Shelves or bits of board to fit Ikea Billy bookshelf (Tilekiln TN38)

I have an Ikea Billy bookshelf that needs more shelves as I only have two of the moveable ones. Colour is not important - they are 77 x 26 cm and do have two cutouts on each end where they drop onto shelf supports but is not important (it depends on the vintage of the Billy shelves) as I have a collection of random supports and can make them work. Any number from 1 to 4 would be great, and also any random bits of wood i could cut down. Width is more important than length. ie a bit longer is fine, or a bit narrower, Julia