The standard Prusa i3 Spool Holder is a wonky and rather useless assembly of threaded rods joined together by a loose bar and some hope.
This alternative holder is better than most as it only has four 3D printed parts, two side cheeks and two rollers. It uses four cheap 608 skate bearings, two in each roller - these should be pressed in using a vice. The spindles for the rollers are a pair of 110mm long M8 coach bolts and nuts (try B&Q). Print the rollers on their small ends and make sure the bottom bearing sockets are clean of loose filament.
Fits are deliberately tight (tap in) to give a good, rigid, assembley, but those who don't like things too tight can increase sizes by 1-2% in X and Y. Don't overtighten the screws or the rollers will be too stiff.
Sizes suit a Factory 3D Prusa with a 6mm thick frame, but just stretch the clips to suit your printer. Holes in the bottom of the clips can be used to take a loop bent out of a TIG welding rod, but experience shows this is unlikely to be necessary.