This is an old revision of the document!
2x http://openbuildspartstore.com/688z-ball-bearing-8x16x5/ 4x http://openbuildspartstore.com/openbuilds-solid-v-wheel-kit/
Or (up to 1740mm) http://us.misumi-ec.com/vona2/detail/110300071070/?Brand=MSM1 http://us.misumi-ec.com/vona2/detail/110300070820/?Inch=0