YouTube member Sunny01 shares a a simple DIY roller bearing slider most people can build out. A few T-shaped joining brackets on each side, some carefully spaced bearings on top and bottom allow the carrier to ride over the PVC pipes. If PVC isn't your thing, there's no reason you can't redesign around some lightweight copper piping. [Thanks Sunny]. Judging from the build and how it maintains itself on the rails, you'll need a minimum of 12 skate bearings. Normally these bearings come in packs of 8.
Your inquiries have been answered. Not long ago Greg Royar mentioned on this blog about a DIY DSLR bearing based slider he made. Here's the video explaining how it's all put together. Of course Greg does mention he's had some help from his Machine Shop to get everything a bit more accurate. Sounds like you'll all have to pressure Greg in pricing out a quote to get a few more of these things run off for sale. [Thanks Greg]
Details from the video:
The slider was built with some simple C-Channel aluminum acting as a rail, and the block is solid aluminum that's been ground down and bolted together. I picked up a box of replacement skateboard bearings from the local sports shop. The bearings were $8 for all 8. Right now I have the slider bolted onto a saw horse from home depot, but I'll probably be buying the popular fold away version so that I can get dolly shots from the ground.