You can find them on ebay sometimes (i.e. rarely, at least the packs are rarely on ebay - seems to be some accessories lately). To find them new, Yes you pretty much have to go to their website. And then my understanding is that you still have a wait of several weeks for them to make your pack.

I'm planning to get a Siwash (from the hunting side) in the next couple months. Just gotta hide another $120 from the wife <img src="/images/graemlins/grin.gif" alt="" />.