This is one of my simple and favorite items that I sell in my shop. You could buy one or learn to knit your own.
Size 7 knitting needles
small amount of yarn
CO 8 stitches
Knit in garter stitch until piece measure 18 inches.
Hold piece around your head. Select where you would like your knot. Usually in the front, to the side, is where I put the know.
Now your piece is smaller. Continue knitting again until piece measure 18 inches again or until it fits your head.
Cast off stitches. Sew together ends of headband.
Questions- ask! It's pretty easy and could made all different sizes of widths.