I have looked at a couple of sites but can't find any of the rectangular or circulsr white number plate stickers. Any suggestions on where I can get a set?

Any vinyl sign makers will cut you a set whatever shape you require.

Ebay is your friend!

Link

Go to B and Q > buy white vinyl/fablon roll > mark desired shape and cut out > job done

[Edited on 31/5/10 by andylancaster3000]

Demon Tweeks do them cl icky

+1 for Tweeks if you're talking about the 'background' for competition number
PS they also sell packs of competition numbers, much cheaper than buying them singly (if you're going sprinting your number will/can change from event to event)

[Edited on 31/5/10 by norfolkluego]

Try Rick at Highgate House, he did mine on my locost race car. He knows all the regs and fonts etc.

http://www.highgatehouse.co.uk/