Assuming you have the regular Discover and not the Miles card . . .
The only gc to gc I know of are:
Best Buy, Walgreens, Gamestop (maybe?), Target. Don't know about BBB.
It looks like you could use your rewards to get a discounted Game Stop gift card ($45 in rewards gets you $50 Gamestop gc). Then you could use the Gamestop card to purchase Ebay. Then eBay to either Best Buy or Walgreens; and then finally to Disney. This is highly YMMV since it seems as if Gamestop is hit or miss with folks (seems like more miss lately) regarding gift card to gift card purchases. Plus, your store may not carry eBay cards and you could be stuck.
You might want to consider just using your Discover rewards at another merchant where you can get a good value on the redemption, and then use your saved cash to get Disney cards elsewhere. For example, I save up my Discover rewards for Shutterfly gift cards. Then I use those to purchase Christmas gifts (a $40 Discover reward = $50 Shutterfly gc). The money I save on Christmas presents goes to my vacation fund. Good luck!