The airstone wand kit I got from Williams Brewing actually came with that filter. I'm not using it since I'm going with pure O2 and no filter is needed. I recently ordered just the wand, since I have a second O2 flowmeter regulator and plenty of tubing to connect it to the wand. Gets me to where I want to be without extra hardware that's just hanging around now (as with the first kit).
A quick vendor search shows that you can get them from both Williams Brewing
and Austin Homebrew Supply
for under $10 (plus shipping)...
Personally, I like the simplicity of the pure O2 setup. Not to mention, no electricity required, so it's ultra-portable.