Filler screws are easily available at gun shops. Both of the ones I use have lots of them and give them away. Most of the time they remove them from guns when mounting scopes and keep them. Supply far exceeds demand so the give them away. Screws for securing sights I have to pay for, and have to go to the gun store for as well. Hardware stores there have a great selection of carriage bolts, but not much in small, fine threaded screws. There is only one sorce that I know of for locking bolts, and you identified it.