Lynxmotion offers a kit with four (4) bronze-bushed rollers with imperial threads for 8.50$. These rollers are made to support the linear motion of shower doors. They detailed a potential installation in their forums, here is a photo:
An alternative is to source them yourself. CRL (CR Lawrence) manufactures most of North America's shower rollers. They offer a 100-pack of rollers for some pretty deep savings. You can find them at a bunch of places online (CRL part # M6000B), here is one place:
This website specifies that the nylon ball bearing roller is 3/4 inch (19.1 millimeter) in diameter with a 7/16 inch (11.1 millimeter) threaded hex hub. However the CRL website mentions a 8-32 threaded hex hub. Which is right?
CRL also offers a 4x.70 mm (M4) threaded metric hub 3/4" oval profile roller but the price is significantly higher.
Another option is to run straight ball bearings as your rollers. Makerbeam EU has a kit that could probably work with Microrax:
This allows for easy CNC work or other moving assembly construction: