hobby lobby has them, when I went in tho they were out of the circular ones so I had to build my own. It works either way but the circle ones would make it easier, I'm not sure on what diameters they carry.
Why do you need these? I run mine with the screens it came with. Ditched the sponges to avoid detritus collecting. The pellets will fit through the holes, but I just run the water too slow to tumble them until they soak and separate.
Bio pellets will run six months or more and tumble. Carbon should be changed at least monthly and should be packed. At least to my knowledge anyway. Running them on my 75 for almost a year. Used all the sponges in the carbon reactor last change to lessen the amount used. Still using GFO on my cube, but that may change when I use it all up.