I calibrated my click type wrenches myself. They are easily adjustable, usually with a socket head screw inside the handle.
Easy enough to make a test rig ... I use a 2' lever and a 50 lb weight, simple math. Also check them with a load cell type torque indicator. Pretty easy to get them within their stated accuracy range. Course this is for my own benefit ... we don't need no stinkin certificates
But I guess if you typically require enough accuracy to warrant building your own test rig .. you probably should be having them professionally calibrated .. I just did it to see if it could be done.