Several sponsors of RDO sell them (MarineDepot.com, ExoticFish.com, etc.), as well as majority of reef hardware places and speciality place like SpectraPure.com and even ebay (surprise!).
I bought a cheap membrane online. After battling phosphates and silicates, I am back to the not as cheap spectrapure filters. Don't waist you money on cheapies, it's worth it to buy quality products for the things that will impact your tank the most.
My 2 cents
Derek
don't monkey around with the cheap filters nothing but a unessary problem will arise. save yourself the hassel spend the money now or you will spend much more later on trying to fix what went wrong trust me i've been there.