I don't have a ready source of screws. I'm somewhat blessed with a large assortment of locomotive screws from many brands due to doing lots of repairs and upgrades. Frequently, you remove things that aren't going back in, and you end up with screws. Whenever an extra screw shows up, it's dropped into my screw assortment.
For certain screws, the long ones to mount MTH tether connectors on brass locomotives come to mind, as well as nylon screws, I do indeed order from Micro Fasteners.