I get my tyres from a UK shop.
[link=http://www.formbymodelshop.com/wheels-tyres-inserts-glues-debondersmisc-18-c.asp]Formby Models[/link]

But am sure you get them from
[link=http://2wheelhobbies.com/index.php?cPath=27_43]Two Wheel Hobbies[/link]
In the states....

EDIT:
With the PMT tyres, you also need the inserts.... forgot to mention that.

Hope that helps and have fun,
Drew