Last year sometime I found an 8mm wrench on the Sloan Ave overpass, where it goes over the train tracks. A nice chromed one, 8 mm open at one end, ratcheting box at the other.
Yesterday, going over the same bridge, I found another one. Different brand, but also an 8 mm open and ratcheting box end wrench. Next time I'm hoping for a 9mm or a 10mm.
Maybe try a larger bridge?