I second the above comment about learning to make a good unbacked bow before you invest time learning to back with sinew, or other things. I started by backing most of my bows, and looking back I believe that decision really handicapped my learning. If I had it to do over again I would focus on making unbacked board bows until I could reliably turn out a solid shooter, then I would start playing with staves and backings and such. When folks come to me asking for mentoring in bowyery, this is the track I put them on.
That said, I have a bunch of whitetail back sinew and would be happy to trade some if you want to augment your stash. I have a great relationship with a local game processor who saves the back tendons from the deer he butchers for me. I do a very good job cleaning the meat, fat, and membrane off it, so it is good clean stuff that bonds to wood very well.