They're pretty common in the 3D print industry. Also check out IGUS, who also do a lot of custom rails/rods. They're probably most well known for their plastic

They're pretty common in the 3D print industry. Also check out IGUS, who also do a lot of custom rails/rods. They're probably most well known for their plastic Drylin bearings. The prices are also reasonable for what you can get. I've been meaning to get some stuff from Makerbeam too, who make 10x10 mm extrusion at a good price (not sure who else makes small width stuff). Oozenest in the UK has a good range.