Wool blankets are readily available at St Vinnie's or Salvation Army shops. Don't be put off by surface dirt or animal hair, after a heavy duty hot wash (and line dry) the blankets come up fresh and soft.
Work out the dimensions of your cushion inner. Measure this out onto the blanket, adding a 1.5cm seam on all sides. Cut out two pieces of fabric with these dimensions.
Pin blanket pieces together with the right side facing in, sew round the four corners and three sides, leaving an opening in the centre of the remaining side to stuff your cushion. For cushions with a firm fit, make your cover slightly smaller than the inner.
Trim seam allowance and cut across corners. Turn right side out, stuff cushion, fold edges of opening inside, pin and slipstitch folded edge across the opening to complete your cushion.