OK, here we go again. I've been racing on and off for over 40 years and have managed to make it without a number on my back. I know why they're now insisting on it but in this era of transponders it seems to me to be a bit superfluous and a few decades too late . My whingeing isn't going to change things however, so where can I get numbers put on my jerseys (screen printed, iron on??) It's more the inconvenience of having to get it done that bothers me more than the actual number itself.
Back numbers are required at club level in Vic so we have them anyway, but what happens when you attend the Nats and get a different number? Buy another jersey so you can put your "nats" number on it? Why Why Why? As Mark rightly pointed out, we have transponders at the Nats now so having back numbers is all a bit too late.
There are iron-on blank sheets you can buy online. Trace your number(s) and cut them out and iron them on.
I think a lot of sports wear shops sell individual iron-on numbers as well.
You can also use a square of white cotton (an old sheet or tee shirt) and paint the number(s) on with fabric paint. Then it's just a matter of securing to your jersey....hook and loop tape (velcro) works and is also available as an iron-on.