You get it automatically when you are on the Dining Plan. If you are staying on site but not on dining plan, you may purchase one to have. They make a great souvenier!!
2 big things to remember:
1-They are refillable at any Disney resort
2-They are NOT refillable in the parks
Keep this in mind if you are planning to refill and take on the bus with you. You will need to carry around with you all day. With that being said, you could ask for a cup of ice water and put into mug. Another option is to use a D-Ring and attach to stroller, back pack, etc.