Here is the best place for them. I think I mentioned it above.
http://www.pagermotors.com/Products....category=Pager
Now the 4mm they say are longer than 12mm, but they are measuring the shaft too; it and the surrounding collar slips through the opening perfectly and You can shut the tail assembly completely; locking it in tight. You may need to shave or grind .5mm from the inside for the length of the can as it is a smidge over 12mm.
They are available without the weights; which is what You want because You risk bending the shaft trying to remove the eccentrics trying to save a little money.
Gary