Hey Bocajan, interesting you ask this. I was reading up Marty's post about Fruit Fly Culturing & he had a couple of tips on finding excelsior, maybe this will help...
It's expensive if you buy it at a craft store. It's cheap if you get it at a packing place of go to home depot early spring when their water fountains come in Don't ask, just take an empty box with all the excelsior you need and tell the cashier it's empty- they never seem to mind. I did that for a long time, then I needed it in the fall, home depot didn't have any, so I bought a bail of it for about $50. That was 2 years ago. I gave a ton to other people and I still have enough for the next 5 years! Get few guys, get a bail and your excelsior problem will be solved. I don't remember where I bought it tough...somewhere on the web at a packing place.
Thanks. Coffee filters sound like the best bet. Cheap and easy to find and no mess when you shake flies out. Now that I know what it is I can make a choice.