Dont bother with those cheap crappy ones linked above on ebay. You want HUBCENTRIC spacers.

I fit these to my ERST:

http://www.burtonpower.com/wheel-spa...2mm-spt12.html

I also did a stud and nut conversion while I was at it. I used 60mm Studs, which are longer enough to clear the spacers and alloys.

I have no rubbing and the wheels fill the arches nicely.