Can the Cadbury Creme Egg's filling be bought independently?
The keyword you are looking for is "fondant".

Fondant is a soft candy characterized by a smooth texture that comes from small sugar crystals.

It can be as simple as sugar and water boiled to the right temperature and allowed to cool undisturbed. Kind of like fudge without the fat and chocolate.

The filling of Cadbury Creme eggs is called "poured fondant" vs the more solid "rolled fondant" that is often used to decorate cakes.

There are many recipes for fondant online and it can be purchased premade.

See this question:
What is Fondant?