Where to get bulk GBC soccer balls?
You should research if there is an RLUG (Recognized Lego users group) active near where you live. These are groups of LEGO fans for which the LEGO company provides various levels of support. 

Often they have the means to buy LEGO directly from the LEGO company through the LUGBULK program. The program allows the members of the LUG to buy LEGO in bulk directly from the LEGO company itself at a discount rate.

I have in the past succeeded in purchasing a reasonable amount of soccer balls this way (among various other elements). 

However, there is only a narrow time frame in which you are able to register for the program and it takes nearly a year until the lugbulk is delivered, sorted and sent to the various members. I'm not sure if the registration is still open or already closed. At least the final selection of items hasn't happened yet AFAIK, so perhaps your RLUG can still select the elements as a favor...