Cambond via Amazon offers the Cambond Fitbit Alta Milanese Magnetic Band in Silver for $16.99. Coupon code "V8NLXAP4" cuts it to $12.49. Plus, Prime members grab free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $6.07.) That's the lowest total price we could find by a buck, outside of the mention below. It's adjustable and features a magnetic closure.
A close price: eeekitonline via eBay has it for $12.83 with free shipping.