I could be wrong, but I thought it was like sandpaper, the higher the number the finer. I thought 400 meant 400 holes per square inch. 250 meaning 250 holes per sq inch, Right? or am I a jackass? I very well could be wrong, lemme go look----
EDIT----nope, I am in fact a jackass, its the opposite of what I thought.
If that's the case, you can just get a cheesecloth, or I think the organic site KIS, has bags with all the stuff in it already (bio-accumulators and such) you could buy one, and then just re-use the bag.
or people have used womens pantyhose too.
I'm not really helping here, sorry I just throw it all in the water, and I don't strain after either, I topdress with the muck