Where to buy CNC parts

OpenBuilds Part Store sells V-slot aluminum extrusions and associated belts and electronics.

Filastruder sells high-quality linear rail hardware separately from their well-known Railcore kits. The smallest belt width they sell is 6 mm.

Bolt Depot sells bolts and screws.

StepperOnline sells many kinds of stepper motors, included closed loop ones.

Adafruit sells a linear rail and carriage, but doesn't have a wide selection for CNC parts.

McMaster-Carr has everything and ships from the US, but it's expensive.

Fermio Labs sells Voron 3D printer kits but has individual linear rails and frame kits available, from Germany.

Bulk Man 3D sells extrusions, rods, and rails for CNC milling applications, from China, with pretty expensive shipping.

Ooznest is based in the UK so it might be difficult to order from, but it seems to have a good inventory.