i did a short search yesterday and only found one manufacturer of artificial sinew. They are in the US, my guess is most re-sellers even the native Americans in Canada sell the same stuff made by the same company.
Artificial sinew is made to be pulled apart and used just as the real sinew was also pulled apart for different sizes of thread so getting sinew that doesn't have that property is going to be hard.
Native American used real sinew for everything from backing on bows, cordage, to sewing baby mocs as well so it was separated to the size as needed for the particular project. artificial sinew is also made so it can be separated.