It could be any ball, really. I don't think it was a pack of regular balls, since then he would say, "oh, my pokeballs are here...."
I also don't think it was the GS ball, since they had the whole drama of trying to get it to Oak which lead to things that are unmentionable.....
Maybe it was a Masterball prototype? Or maybe there is simply a company that will make fun pokeballs for children and people like Professor Oak?