Roy, i've been looking for these as well. I dropped one and it's long gone. i was using cotter pins for a while, then a piece of rebar wire. then i gave up and don't bother using anything. Ive bladed a few miles of road, mowed a bit with the brush mower and have yet to loose a pin. I'm likely just lucky. At least i have some luck somewhere....
I find i was pulling those pins almost every implement change. Gotta have something quick and easy.