i was doing some research on recurve bows / composite bows and began wondering the same thing. I live in Melbourne Australia, and it's rare for me to see butchers even stock kangaroo meat. From what I understand it's an acquired taste.
Perhaps out in the country areas it might be a bit more common.
Anyway, the sad part is that in some parts of Australia Kangaroos are seen as a pest and killed on mass we're talking millions.
I'm not sure what they do with all the by products but i'm guessing the most valued parts are the meat and hides. But yeah, there's probably plenty of left over legs and tail to get tendons from. Which you could probably make you own sinew.
You could try some of our government agencies, they usually have professional shooters and station owners who they issue licenses to and permit to kill. Maybe they could refer you to someone and you could arrange for them to strip off the tendons before they grind it up with the bone to make fertilizer?
Have a look at the quotas they've put out and this is only in one of our states
http://www.derm.qld.gov.au/wildlife-ecosystems/wildlife/wildlife_permits_and_licences/kangaroo_harvesting.htmlAnyway hope that helps
- Devon