your local hardware/home improvement store.
Its called expanded metal. I got some for my truck that is much finer than that (ie, it doesnt LOOK like expanded metal) for about $1.76 at Home Depot. the stuff I got was actually for covering gutters to prevent leaves and sticks from clogging them, so look for "gutter gaurds" if you want the small stuff. I always get people who ask me how much i paid for my grille.