/u/judahloewben is partially correct, one non negligeable source of vitamin C would have been liver. As to "...the Inuit, who lived in the treeless tundra, avoided scurvy by eating whale skin." that statement has to be corrected, as the vitamin C in the Inuit diet is from a diversity of sources, including liver, from seal, fish and verious other animals as well (with a seasonal input from berries and a few herbs in summer) (see: Geraci and Smith, 1979). They did (and still do) however eat the whale skin (with pretty much every other part of the whale as well), but it is an infrequent component of their diet (at least today).

Back to the plains. You've got to realise that the plains were not a homogeneous environment and that lusher vegetation can be found along rivers and streams. Plains indians also had access to a variety of berries which provided vitamin C. One of the most important was chokecherry (Prunus virginiana, see Montana native plants and early people, Hart, 1976), which was used in various ways, and could be dried and was notably added to pemmican. Wild plums (Prunus angustifolia) also were used, as were currants and other assorted small fruit.

Another important vegetable food source was "prairie turnip (Psoralea esculenta") which was established to be a non negligeable source of vitamins C and A. There is abundant littérature stating the importance of Psoralea in the dietatary input of plains indians (see for example Wedel, 1978)

Almost all mammals can produce vitamin C (humans and Guinea pigs are two exceptions). While muscles (Ie meat) doesn't contain vitamin C some internal organs do, mainly the liver. So while I can't say exactly what the Indians ate, it is entirely possible to get all the vitamin C you need from eating mammals.