You can sometimes find them on ebay, but often they are listed as 5C sleeves, and frequently are listed as fitting a particular lathe. You might be able to find the specs on the lathes listed and find one with a MT5 taper in the headstock.
As for the thread sizes on the internal and external 5C collets, I have discovered they may not all be a standard size. I have a 5C collet block and a depth setting fixture that is supposed to fit the inside thread. I have found some collets fit these better than others, and some of the internal threads are too tight to fit the depth fixture. I do not understand why they would not be a standard size.
