GJW,
It would be easier to help you out if you give us an idea of the scale you're working with rather than the desired finished dimensions because it makes a big difference with the finished product. There's a lot of grass mats available to the model railroading community, including these offerings from Walthers:
http://www.modeltrainstuff.com/Grass-Mats-s/2835.htm
It works pretty slick for HO scale, but for something like 54mm (1/32 scale) figures, you're usually better off making your own grass for the diorama or the sense of scale is skewed.
And just a heads up from a Montanan who has wandered that battlefield many, many times over the decades - the soil there is an ashy grey that is pretty distinctive. Like any other open field in eastern Montana, it looks a lot like a solid mass of grass from a distance. Once you're standing on it, you see how the grass grows in random patches. The early photos of Last Stand Hill (taken three years after the battle) show a lot of open spaces between patches of grass and is very different from the photo you have above. The battlefield guides have commented that wildfires during the 80s burned up a good portion of the field and allowed "invasive species" such as cheat grass and Canadian rye grass to take over things as they explain how hot, dry, and dusty the conditions were there in 1876. Personally, I would err on the side of caution and try to recreate what I see in the photos from 1879 rather than contemporary photos.