You might find somebody down there that can dig out the flag, but I wouldn't bet on it. I had high hopes for getting it on loan for a museum exhibit and had no luck. I don't even know that that it was an Arkansas flag (much less that one) that was once on display.
Several reports indicate that Santa Anna took more than one battle flag back from Buena Vista, and some reports indicate that one was an Arkansas Mounted company flags. But are they still there? No telling.