You do not have to get just Paizo products, but due to Paizo’s PDF restrictions you will need to own the Paizo PDF if you want to replace it with a Paizo product. Other than that you can get whatever you want that’s comparable in value.
Maidhc, as I said in the update and on the form, you may choose other combinations if you like. If you really have your heart set on two bestiaries I suppose I can fix it for you since it hasn’t been issued yet.
