You can buy them through orbitz. It is a good deal for some character meals depending on when you go (like Goofy's for dinner) but not a good deal on others (like Goofy's brunch). At least that is the case for our trip
If you use ebates to purchase from orbitz, you will get 3% back. I haven't seen any other discounts, every site charges, for example, $33/adult for the regular character meal voucher. The nice thing is that the vouchers include tax and gratuity.
We got free character dining vouchers as part of our Costco package once. Now we have AP's so we get 10-15% off depending on the kind of pass (now we have premium so we get 15% although I keep hearing the AP dining discounts could go away).