Groove Guide Pulley 8*40*20.7mm 0840UU

I 3D modeled this exact pully.
http://www.banggood.com/8mm-U-Groove-Ball-Bearing-0840UU-Groove-Guide-Pulley-Sealed-Rail-Ball-Bearing-84020_7mm-p-1046816.html

Exact one used in DiyPerk's how to make a diy camera slider

All you need is ball bearings such as these:
http://www.ebay.com/itm/10-Bearing-608ZZ-8x22-mm-608Z-Metric-Ball-Bearings-VXB-/400073187613?hash=item5d2638611d:m:mgY3Tje8TUIMghpOEnOYl3A