With two of these, a box of #6 washers (http://www.mcmaster.com/#90126a509/=su437y or your local hardware store), some 2" split key rings (http://onestop-key-rings.co.uk/-p-11.html or ebay), some mounting hubs (http://www.pololu.com/product/1079), and some #2-56 screws (http://www.pololu.com/product/1957) you can make your own omni wheels suitable for RoboCup Junior competitions easily and inexpensively (approximately $15 plus shipping for a set of three).