To my knowledge, there are only two places where you can get "Modder Mesh". MNPCTech had some great honeycomb and round sheets. Unfortunately, they are sold out and only have 1x1" round sheets left. The other modding mesh I found is sold by AC Ryan as MeshX. However, this stuff is strong but VERY restrictive... I would guess less than 30% airflow. MeshX is no good for covering fans.
Most companies only sell perforated metal sheets in bulk and I personally don't think I will ever use 20-100ft of the stuff. I've been looking around and finally found a site that sells to end-users. They sell in 1x1" and 1x2" sheets. I also recommend using 22-18 gauge thickness and go for the aluminum or stainless steel. Stainless Steel Grade 304 is the most common kind you will find. Grade 316 is better at corrosion resistantence in chloride environments. Plain steel requires sealent or paint to prevent rusting. Plastic is ugly... but whatever floats your boat.
Aluminum: www.smallparts.com/products/descriptions/pma.cfm
Plain Steel: www.smallparts.com/products/descriptions/spm.cfm
Stainless Steel: www.smallparts.com/products/descriptions/pmx%20and%20pmxx.cfm
Polypropylene (White Plastic): http://www.smallparts.com/products/descriptions/2g1.cfm
PVC Plastic: www.smallparts.com/products/descriptions/670.cfm
They have a few other interesting PC parts:
Hose clamps (cheap): www.smallparts.com/products/descriptions/chn.cfm
3/8" Coupling Connectors: http://www.smallparts.com/products/descriptions/2170.cfm
Large Neoprene Sheets: http://www.smallparts.com/products/descriptions/892%2Ecfm
Acrylic Sheets: http://www.smallparts.com/products/descriptions/las%2Ecfm