I just went out to the Rockler web site and it looks like they don't carry the rollers any more. But, I found a source here: Bed Box Rollers | Woodworking Showcase
Yes, I do have the gap between the back end of the kitchen and the front of the bed. If you look at the photos that I posted earlier, you'll recognize it. There seems to always be something dropping in the gap. A filler is a good idea.
I thought about drawer slides for the under-bed drawers, but they would have to be super long. When I went looking for the super long ones, the prices were very scary. I've since thought about pairing shorter drawer slides, and somehow mounting them to a piece of wood such that when one is fully extended, the second one in the pair would extend. I haven't worked it out completely, but I know I could buy two dozen or more 22" slides for the price of on set of 44' slides.
With the rollers, I don't worry to much about tracks. The drawers roll in pretty straight and if they're a bit crooked, I just nudge them where I want them to be.