Give the school office a phone, I'm sure they'll be able to tell you where you can get them.
I remember when I was wee, going with my mum to a shop to get school ties and the like, it was a mens clothes shop but it stocked all the school stuff for the area, you probably have a shop like that near you. Other friends remember going to shops like this, they were usually called 'Mansworld' or something, and I remember thinking it terrible that I had to get things from a guys clothes shop!
But first stop I would say should be the school office!
Good luck!