Certain items are only available for purchase via Mobacoin, which is obtained by spending real money on the game app. The current rate is around 100 Mobacoin per $1 spent, and they are available in quantities of $5, $10, $25, and higher denominations.
Some Mobacoin-only items are better versions of existing items, but they also can shorten Adventures, get into special areas, and expand your Ranch.