I think the part number you're looking for is 6383K12 (for 1/4"). You can search for "Steel Semi-Precision Ball Bearings" or catalog page 1014. I know that when I started I had the same question and answered it by reading Jgro's original thread. Its near the end but he also give a part number for the aluminum channel.
I'm pretty sure that UHMW can be substitued. I think some people have even used plastic cutting boards.
I hope this helps.