Where can I get plastic rollers

I saw a picture somewhere of a 2d drawing of a gantry.

It had some rollers that rested on a round bar. The rollers were
essentially wheels with a semi-circle grove on the rolling surface, so
that the wheel would rest nicely on top of the bar. Making it an
effectively it a linear slider.

What I'm looking for are the plastic or nylon rollers. I could just put
a slab of plastic onto a lathe and machine them myself. However, if I
can just buy it, I'd rather do that.

I've seen them used in tools from the hardware store, so I know that
they exist, just that I don't know where to buy them.



--
Lloyd Leung
Lloyd@...