There is also of course the opportunity if you have a car to take a drive down to the Crossroads Lake Buena Vista - there is a supermarket in there that does sell balloons and also amazing cakes. We use it all the time when celebrating as they do also deliver to the Park Restaurants and Hotels - at a much cheaper price than the Disney Parks / the Disney Floral and gifts.
If you were to drive down to Disney Springs and continue past the Wyndham and on Disney property other resort hotels to the end of the road then go straight across at the lights into what looks like a shopping area - the supermarket is right there. They do all Disney Balloons - however I haven't seen the clear ones that have the mickey head In - but well worth a visit - its the cheapest and nearest place we have found - its even good if you run out of sunscreen.