Do not spend your currency to purchase new Wooparoos. Instead, breed the Woopacross you already have that are at least level four to get a rare crossbred Wooparoo. When breeding Wooparoos, you will be able to tell if you will be getting a rare variety by checking how long it takes to breed and the time that the egg requires to hatch. The longer the time, the better the chance of a rare Wooparoo.