If you've despaired of ever finding a bed for your pet made of anything but corduroy or sailcloth, despair no more. Now available are beds for dogs and cats that are made of 100% cotton or polyester "satins" and "silks" in a variety of colors, styles and sizes, and they're machine-washable as well. Custom-made beds covered with your choice of fabric can be ordered by writing to the manufacturer: Designer Pet Products Inc., P.O. Box 1867, Hollywood, Fla. 33022. Selected ones are available at the Bowser Boutique, 610 N. Robertson Blvd., Los Angeles, and at Beverly Pet Center, in the Beverly Center. Prices range from $35 to $100.