I make dog/cat beds by stuffing pillowcases that I pick up at about 50¢ each at the local Thrift shop. I use scraps of fabric and batting and don't worry about the sizes since the animals like to "adjust" to suit themselves. Everything is washable. I have also made larger dog beds from cotton drapery material that I was given. Stuffed them with mostly batting otherwise they got heavy. Again washable. I've been supplying the local vets office and the local Animal Rescue.
If you make it know what you are doing LOTS of people (quilters especially) are happy to save their scraps and fabrics from sorting their stash. Right now I have about 9 bags of stuffing for pet beds plus some pillows and old comforters and blankets.