Yes, Polyfilter is the stuff I was thinking of. You're right, it is expensive but absolutely worth it. I'm not sure what I paid for a small sheet of it, but I cut it into 1-inch by inch cube or slightly smaller and put them all in an airtight bag. I put one cube into each 1-liter Pepsi bottle with one apisto, with about a third or quarter of the bottle filled with water and one leaf (from litter) for the fish to hide under. Obviously, no oxygen. The fish were fine for more than a week and if the Polyfilter turned dark brown, I replaced it with a fresh cube. I'd take two 100-quart Coleman cooler down with me and these hold a LOT of Pepsi bottles. This works really well for killies too. If you are going to ship Apistos across the country, this technique is certainly worth looking into.
Is 'Polyfilter' the word you're looking for?