I have spent several days searching for a reliable seller of air filters and inquired at leading industrial suppliers, but as it turned out, these industrial suppliers are also manufacturers of industrial grade air filters so they snub small time buyers like me. But assuming they grant my wish to buy small piece of their product, still they insist to custom build the air filter with frames and their minimum thickness is about 1" which is too much for a small air purifier like what I did. But yes, I really like the HEPA air filter they are selling because it is the world's most efficient air filter.
I haven't gave up on them yet, coz I'm so crazy about their HEPA air filter, but it is nice to have, but with the kind of supplier, well, oh well, I am still hoping that I could find one and instead I ended up with air condition air filter. It is a 3M brand called "Filtrete" .
One particular feature of this product is it cleans the air and very efficiently because it used electrostatic fibers which act like magnets to trap dust, bacteria, dust mite, and many harmful dust particles. Here's the actual air filter:
The package recommend just one layer but I used 2 layers. SO that makes 4 layers of air filters inside my air purifier. Why 4 layers?
4 layers because I used eco bag to wrap the charcoals and activated carbon so that makes it 2, then I added 2 layers of Filtrete so that makes it 4: