Buy whatever biological media they have at ur lfs (bio max matrix api chem stars etc) the fx5 holds 1.5 gallons so buy a lot! 4-5 boxes, rinse under warm water and put them in each tray. Make sure all the baskets interlock and ur good! the huge sponges take care of the mechanical filtration!
Cheapest bet is buying in bulk online and finding a deal on shipping. You can buy Seachem Matrix in 4L containers. Try Amazon, Marine Depot... You probably need about 6L.