Sorry to hear, the same happened with my first speedy purchase. If its that important, Keep trying!!! They gave me the same run around and finally, after going through no less than 5 stores I found one where the kind manager gave me one no questions asked. I know for a fact that they do have extras. I don't know why most SAs are so reluctant to give customers the size needed, it is not as if we are asking for an extra. Good luck! I'm sure if you keep searching you will find a nice SA.
In the future, I recommend you ask prior to committing to a purchase. For my most recent speedy purchase the first SA who helped me told me it was absolutely not possible to get a larger dustbag instead because "...we only have one dustbag that comes with the bag and we don't have a single extra." I walked away. The next SA that helped me gladly gave me the larger size instead. SA tend to make up their own rules. Just this past week I asked a SA for a larger box for my wallet purchase and she told me, "I can't do that. We have only one box and it goes with it." I told her they had done it for me in the past and she just said, "The policy has changed." I went to another store and the SA there happily gave me the longer size.