I've taken to making my own. I chuck the bits in my drill press and use a mill file to cut down the corners of readily available commercial bits of whatever length I desire. After a few passes, the bit will slip into the Centrotec chuck.
Then, I put a metal cutting abrasive disk in my table saw, chuck the bit in my hand drill and grind the notch that is used to lock the bit into the Centrotec chuck.
Charles