You can make a Mad Hatter hat from a bit of fabric and ribbon, some wire, and a little of your time. To add some verve after the hat is finished, give the hat brim a "crease" in the front and back at a jaunty angle.
Making a Mad Hatter hat
Measure the circumference of the wearer's head and add 1/2 inch. Cut a piece of wire to that length. Cut two more pieces of wire, each 36 inches long. Bend all three pieces of wire into circles. Join the ends of each circle by twisting them together or by wrapping them with thinner wire, such as 18- or 20-gauge.
Cut a piece of black fabric 36 1/2 inches wide by 12 inches long. Mark the bottom edge of the fabric with the length of the shortest piece of wire you cut plus 1/2 inch. For example, if your shortest wire is 21 1/2 inches long, mark the bottom edge at 22 inches. Starting at the top, gradually taper the fabric width from 36 1/2 inches to the narrower width you marked. Sew the fabric together along the 12-inch side, using a 1/2-inch seam allowance. Turn the piece right-side out, so that the seam is on the inside.
Place one large wire circle inside the wider end of the fabric tube you just made, 1/4 inch from the top. Turn the edge of the fabric to the inside and hand-sew it over the wire. Turn the piece upside down, place the smallest wire circle inside the narrower end of the fabric tube 1/4 inch from the top. Turn the edge of the fabric to the inside and hand-sew it over the wire. When you're finished, the wire will be completely covered and you'll have a black tube with a wire circle sewn into it at each end.
Cut a circle of black fabric that exactly covers the wide end of the fabric tube. Place it over the wide end of the fabric tube. There will not be any overlapping fabric; it will fit exactly over the fabric-covered wire. Glue it into place. Let it dry.
Cut a second fabric circle that covers the other large wire circle and extends 1/4 inch all the way around. Place the wire on the fabric circle and turn the edge of the fabric to the inside. Hand-sew the fabric over the wire. When you're finished, the wire will be completely covered and you'll have a flat fabric-covered wire circle.
Center the open end of the fabric tube, the top of the hat, on the flat fabric-covered wire circle, the brim, and glue into place. When the glue is completely dry, turn the hat over and cut out the middle of the flat fabric circle so that the wearer's head will fit through. Leave 1/4 inch all around the edge, turn it upward into the inside of the hat and glue in place. Cut a piece of ribbon to fit around the top of the hat and glue it into place, with the bottom edge of the ribbon resting against the brim. Make a tag from paper, mark it "7 3/8," and slip it under the ribbon on one side of the hat.
Be sure to use glue that's intended for fabrics, and let it dry completely between steps.
If you'd rather not sew, you can use an iron-on fusible product to join the fabric.