Well I have seen a lot of people use pantyhose wrapped around the filter outflow or one person on that thread that I posted used gauze and posted a picture of it... I think these would be easier to find than a filter sponge if you would like to try it.
Otherwise, I believe you might be able to find something that would work in the filter media section of petco. Something like this may work...
Or maybe this? http://www.petco.com/product/102413/...20Media-102413
The second one I believe would work well if you have a hang on back filter where the intake is a tube... You can place those sponges over the tube to ensure that your fish will not be sucked into it. But either one could likely be tied to the outflow to reduce the current. However, I don't know how large they are. When you get a chance to make a trip to petco, look to see what they have.
I also recommend fishing line or something to attach the sponge to the filter (or whatever you decide to do) because a rubber band will erode, unless you can keep the rubber band from touching the water.