MB 224: with the sinew of an ox - and if he doesn't have that he should use the sinew of small cattle [e.g., sheep or goats] but from a non-kosher animal even after the fact it is pasul. It is not clear if one can use the sinew of the vein [gid hanashe] for this purpose. [The sinew of the vein is a sinew that one is not allowed to eat because of the story of Yaakov and the angel (see Breishit, the end of chapter 32). I think that the doubt here is do weu say that when the torah said "so that Hashem's torah should be in your mouth", from which we learn that we can only use something that we can put in our moths, does it mean the animal in general or any specific part that can't be put in our mouth cannot be used for tefillin.]
32:50. One may not buy sinews from a non-Jew because (225) it might come from a non-kosher animal. In a place where sinews are not available one should sew (226) with "talyadrosh" that is made from parchment until sinews (227) are available.
[I will preface this section with a short discussion on majority (rov). If in a town that has 10 butchers, 9 of which are kosher, one finds in the street a piece of meat, it can be assumed that it is kosher, since we say that anything that departed of it's original place, came from the majority (kol deparish meiruba parish). However, if one went into one of the butchers and bought a piece of meat and doesn't remember which butcher it was then, since one went into it's original place, there is no rule of majority and it turns into a regular doubt and therefore the meat cannot be used. The rule here is: "Anything that is permanent (kavua, i.e., in it's original place) is like a 50-50 doubt." Now let us proceed to the Mishna Berurah]