Since Walt Disney World (Florida) started using Magic Bands in 2013, there have been collectors for the different colors and styles of Magic Bands. For more information on the history of Magic Bands, check out this article.
You receive your Magic Bands in the mail prior to your trip if you are staying on property, however you can always order additional colors and styles from the Disney Parks store here.

For the upcoming 45th anniversary of the Magic Kingdom in WDW, Disney is releasing lots of collectibles, including a limited edition Magic Band. Magic Bands are your ‘key to the world,’ including your room key, park tickets and fast passes.
According to Ethan at MagicBand Collectors, it will be released on October 1st at the Emporium in the Magic Kingdom. There aren’t any photos of these special Magic Bands yet, but follow Ethan’s blog here for updates.
UPDATE: Here’s the pic of the special Magic Band

Of course, if you want to DIY-style your basic Magic Band, check out my previous post on decorating your Magic Bands here.
Have fun exploring the World!