We have some M5, M8, and M10 smooth steel rods that are perfect size for most desktop 3D Printers.  We also carry bearings for the M8 and M10 rods Here
 
Most pieces are 40-45cm long, but some are shorter. Please speficy the length you need and we will send you the closest one we have, or you can opt to have us cut it to length for an ectra $1.