I remember seeing this, I haven't tried it myself but I guess the stickers would stick ok, this is what the guy said
I had some stickers made up for plastic helmets for my club in February and used a company called Diginate, based in South London. You can specify the shape, size, etc and do it all online. The only thing you need is a high quality image file of your club badge.
It's a bit of a fiddle to design the badge online, in the end I just sent them the file and some instructions on what I wanted and it was all good.
I had only six stickers made up as a trial run, on square cut clear vinyl 50x60mm and they have been superb. There is no minimum quantity and for the six I paid only �8 including postage.