Can one buy a tail of a calf from a Goy to use for wrapping the Parshiyos of Tefillin, or because it is very hard to tell different tails from each other, we are worried that he might switch it for a nonkosher tail?

 

It would depend on what is mostly available, whether it is known that some use hair from non-kosher animals, and where and how it’s purchased.

 

ראה כעי״ז בנוגע לגידים – אדה״ז לב, עה.