I don't have a problem using leg sinew from whitetail deer. Most deer processors will give you all you want.
I wonder if some guys who have had problems getting leg sinew to lay down well, broke them down into fine enough strands? The more they're reduced in size, the better they are to work with and the nicer the end result. I take them down thin enough that if I try to divide it one more time, it's likely to bust in half as I peel them apart.
Another thing I noticed... some of you guys presoak the sinew in water. I don't do that because from their completely dried state, when they soak up ANYthing, I want them to be soaking up hide glue, not water. The more water that's in it, the less glue it can suck in. If the sinew is in fine enough strands, it softens in the hide glue very quickly anyway. Just my $.02