Hi,
You can buy styro balls from this website, they sell products about Styrofoam https://www.styroshop.com/. I know some of the children requires them to have styro balls, or styro letters. hope this message helps you.
White Star Trading (WASCO) in the Beach Centre has styrofoam balls but I'm not sure what size they go up to. If you need them to be really big perhaps you could contact Styro Foam (? I think that's the name) in Sharjah and maybe they could cut it for you? Might be expensive though... We've used them previously for giant blocks.