I made my own with gutter mesh from Home Depot. It's definitely a worthwhile project. Last time I cleaned my radiators, there was hardly anything in them.
The question of mesh guards for the GT3 bumper has come up before and it appears no one makes them. Even if someone did manufacture them, I can't see paying $100.00 for something you can make for $6.00.
Check this thread for pictures of how it's done.
How much dirt could really be in there?