Just cut down old bed sheets, they will be softer than new! Use Pinking shears to get a wavy edge. Hope that helps
You can use fabric glue to form the corners OR use the sheet holders for a regular mattress that attach underneath the corners and use any folded flat sheet or small fabric like receiver blankets.
use a pillow case
You can also buy fabric to fit and follow other's instructions.
Edges cut on old sheets with pinking sheers, if not hemmed properly with a sewing machine, are going to fray, and will not hold up to the regular washing that baby bedding needs to keep it clean and sanitary. Also, old sheet fabric will not be sufficiently absorbent to be used on a bassinet.