Cream of wheat if fine if you dry it well in an oven at about 130 deg for an hour before using it. Do it each time you set up to reload, as it is hydroscopic, and the moisture it picks up can kill some powder.
If you check the internet I suspect you could find a good price on the plastic buffer though, and it doesn't take much if the case is pretty full of powder, which is where any filler works best. If the stack of filler is too long it becomes a broken bullet base as it exits the muzzle, which isn't condusive to accuracy.
I did a little experimenting many years ago with sawdust, with results that indicated more experimentation was in order, but it needs to be run through a screen so you don't have chips getting in the way. I didn't have a proper mesh screen, and had several container of shot buffer so never tried sawdust again. I think the sawdust should be hardwood, of any flavor, not pine, which contains rosin, which is a super flux, but very very little. It may be an asset or a detriment, I don't know. Don't hesitate to try it if you give sawdust a whirl though. Also, keep your sawdust in a real dry place or dry it like cream of wheat. It can hold a lot of moisture, though it isn't as hydroscopic as the ceral.