@enola: Go to the creatu search page https://www.rescreatu.com/petsearch/
and then you'd want to change the 'location' parameter to Rancher Shops, the species to whatever specie(s) you are looking for, and the color to whatever color(s) you are looking for. Price will also have to be set to 1 to find creatu that are actually for sale and not just unpriced!
And then lastly I always set the order to be by price so I can see the lowest priced creatu of that color