Generally speaking, I think aftermarket companies are going to be hesitant to block off airflow to the radiator for liability reasons, so finding an aftermarket grill block is probably not going to happen.
The shape of my grill made making a grill block a little tricky and it was damaged... so I bought a cheap, eBay mesh grill that gave me a nice flat surface to mount the coroplast to.
Grill block:
You can see the back side of the grill here:
