You can buy polycord from McMaster-Carr, search for ‘polyurethane belts’ on http://www.mcmaster.com.
Many things will work for rollers/pulleys. You can buy them (very expensive), use wood dowels or PVC pipe amongst other ideas.
1/4" seems to be the choice of most FIRST teams, that is what we use. If you get solid core, you will need to be able to join the ends together. There are some great CD threads about doing this cheaply (w/o buying $500-$600 kits). The hollow cord will work fine and is easier to join the ends but I think the barbs (that hold the ends together) force one to use larger diameter pulleys/rollers.