The material for screen doors might work nicely (the metal stuff)
But I would think any home improvement or ACE should have something including the vent screens you mention.
Another thought that might fit would be the vents that are in the eaves of houses to ventilate attics while keeping critters out. Might be a perfect fit.
2015 Grand Design Momentum 385TH
2015 Ford F-350 DRW 4x4 Lariat