You could try these people who make tyre art markers - perhaps they will sell you a tin!

http://www.tyreart.co.uk

Lots on eBay

http://www.ebay.co.uk/sch/i.html?_odkw=t...ge&_sacat=0


New tyres

http://www.ebay.co.uk/itm/Avon-Safety-Mi...=item35d1b9312e


Omne trium perfectum