Filter media is like carbon bags, foam, bio rings (Biomax), bio foam, etc etc. Google Fluval filter media. I buy everything online, so check this out.
The 304 should have 4 foam blocks, and 3 baskets. Everything in the filter needs to be replaced at some point, but not at the same time. If you received a used fluval 304, I would start by buying all new "Media". Any beneficial biological bacteria (and yes bacteria is important) will die if the filter is dry for an extended period of time.
For a start, I would get all new...
4x foam blocks (vertical foam blocks),
All this assuming the 304 will be enough for 100g tank. The good news with fluval can's, most everything can be transferred from the 304, to the 406 (minus the vertical foam blocks, as they change in height). Setup your filter, run the tank for a week or 2. Check water conditions before spending a lot of money on fish or plants.