I haven't been able to get any either - I think its a case of being between one vendor shutting down and the other not yet up to speed - in fact, a lot of the SuperStreets or E-Z Streets stuff is getting nearly impossible to find.
The most common type of center rollers on SS vehicles -- the very tiny rollers with the single-piece combined level arm and spring (left side, photo), are quite delicate. Some of the later step vans and panel vans have a normal type roller (right side - smaller, but otherwise as on a loco) - these are the only ones I think kids should handle. Fix the other type and its only a (short) matter of time before its broken again.
e-mail me if you cannot find a source for spare. Look at the picture below and verify you have the type of roller on the left. I have a few spares I've taken from among the SS vehicles I bash or install the other type of roller on. If you only need one or two I can mail them to you, no charge. If you need the roller on the right, you can get those - they are used on handcars and clown cars . . .