Interesting idea Ales and think the part about raising $ to purchase public land for restoring to native prairie conditions is a good one. Trouble with adopting particular animals in this case is the herd is likely thinned yearly with animals being auctioned off to private herders or farmers, etc...
There actually is at least one sizable organization in place that I know of who are trying to buy up ranch land in parts of Montana, Wyoming, Nebraska and the Dakotas in hopes of creating a massive prairie preserve and I believe the vision ultimately is to install a huge Bison herd. This is unsurprisingly a contentious issue out there with ranch families concerned about losing their livelihood and so forth.
Another thing to consider is the historical significance of the Bison as a food source. There is an author named Dan O'brien living out near the Black Hills who also runs a Bison farm and is involved in field harvesting of Bison working with private individuals including Indians from neighboring Pine Ridge and Rosebud. Essentially the animals are hunted or harvested in the field rather than butchered in a slaughterhouse like setting. Pretty cool scenario really. Might want to check out some of his books (The Indian Agent is one that comes to mind) and also his company website (I think it is called Wild Idea???).