Welcome to the community!
That is pretty darned exciting. Never knew there was such a promotional item from them. Seems like a unique and special promo!
Have you kept fish before /within the last "recent" period of time? Are you familiar with the Nitrification cycle/ cycling a tank, and what beneficial bacteria (aka BB) do for the system? If you need info about that or a referesher, there is a link in my signature for getting started with a new tank.
Do the instructions provide you with a size/gallons or liters?
There are a few options and what is available where you are. A sponge chunk which would be like a replacement part in the store just which size would fit, or cut to fit ( you could get a larger one and cut in smaller size to double the value), may come in blue, white, grey/black.
Ceramic rings look like this, there are different brands, but I would stick to brand name types.
Cobalt Aquatics Biological Filter Media: Biological Filter Media for Ponds and Water Gardens
They would be able to slide into the larger space, in the middle I think it was.