I never really have to collect road kill for fly tying materials. I collect many of them while hunting.
Duck season brings in many mallard flanks and wings, teal flanks, and wood duck flank feathers. I also collect the CDC feathers from all of the ducks, they are the feathers around the oil glands. Geese bring nice wing feathers for biots.
Small game and trapping brings other furs. Fox squirrels have great fur for zonkers, tails of both types of squirrels are used. Lately, my tails go to a friend that uses them predator trapping. In exchange, I will get a patch of fur now and then from muskrats, beavers, skunks, and coyotes.
Turkey season brings nice tail feathers and some marabou.
Pheasants have nice tails and other feathers, I have some from tame ones we raised. Same goes with peacocks, we had a few of them one time and I have a lifetime supply of tails and swords in a large vase. A few banty roosters were scared around me at the farm also, they were plucked from time to time for saddle hackles.
Deer season provides tails and belly strips. Young deer have a great fur on the back. I have been given elk from friends that hunt them.
When washed and put in the freezer for a few weeks, all pests will die, and the feathers and furs will last a lifetime.