There is a small gift shop at the visitor center with the usual trinkets, books, nice photographs, etc. Please do not buy pieces of cave formations from any fly-by-night "vendors" on the way to the park! Even if collected from caves on private property, collecting them amounts to cave vandalism, and in any event, it's a purchase you'll regret soon enough. Within the caves, they are things of beauty; outside, they are just little hunks of rock that are likely to disintegrate before you get them home. Spend your money on good photo collections instead.
