Buying a prexisting track will generally be out of the question if you want CC-BY-SA since it essentially means that track is liberated from restrictions and may no longer be a source of income for the artist. I'm still not sure how Doc got that deal. Maybe the artist is counting on the obscurity of our copy to save him from losing income on it.
Speaking of which, we could totally use this same track over and over from now on, but I think people will complain if we don't change it up.
Custom music is expensive. As expensive or potentially much more than an entire video. Probably shouldn't expect it to be made with FOSS, either as its super rare to find musicians using FOSS solutions over proprietary ones.
That said, I'm fine with using a track decided on or made for the project in future videos provided the quality and mood fits. I'm just not qualified to do make such a track myself. :)