unscrew the washer thing on your sink. You can remove it by twisting it if you do it hard enough. there are usually more than one screen in sinks. i just took one out and have been using it. its basically what you['d buy at a head shop.
Just find a small pebble and wipe the dirt off and throw it in there. It's worked for me several times.
Topnotch: I do the exact same thing. passittotheleft: I'd do that if I found a cool pebble, like a nice blue or red one; but u should try the weed method that topnotch and I use.
Wont that burn? The screens on every window in my house arent metal... I do the bud-covering the hole method too. Only you gotta be careful when all you got left is shake, theres a good chance you could end up inhaling a ton of ash.
window screens melt when burned.. well when torched.. but i usually torch a screen before i use it to burn off any possible residue/chemicals that may be on the screen
If you have nothing available to use as a screen, try foil. Take a piece of it and mold it into a size that would fit your bowl. Poke some small holes into it. However, don't use it on a long term basis.
I did a little research regarding the foil thing. While health problems could occur, it's rare, if it does. Aluminium toxicity is rare and only usually found in patients with poor renal (liver) function. http://www.emedicine.com/med/topic113.htm http://www.emea.europa.eu/pdfs/vet/mrls/039398en.pdf