For screws, check Brownell's Gunsmith Supply:
http://www.brownells.com
You can call or email and they'll help you find what you need.
For attaching mounts, here's the gunsmith method:
Degrease the screws and the holes in the receiver with a good solvent like Acetone or lacquer thinner.
Use pipe cleaners with solvent to get the holes clean.
Allow to dry.
Apply Loctite Blue to the holes and screws.
Put a FITTED screwdriver bit, OR an Allen or Torx bit into a screwdriver handle.
Quickly screw in the screws, and before the Loctite starts to set up tighten each screw down tight, THEN pushing down on the driver AND continuing to turn the handle firmly, give the handle a sharp rap with a plastic hammer or the butt of another screwdriver.
Not TOO hard, just a sharp rap.
This will force-match the threads in the holes and the screws for a perfect fit, AND will rupture any air bubbles that are in the holes.
This insures a tight, no-loosen mount job.
Do all the screws before the Loctite sets up.
Allow 24 hours for the Loctite to fully cure, then apply a lube to the mount area to prevent rusting.
Some people use enough Loctite to leave a bonding layer under the mounts themselves.
After the screws are in per the above method, apply more Loctite so it will wick under the mounts and completely fill any gaps between the mounts and the receiver.
This helps prevent vibration in the mount from loosening screws, and serves to seal the area from moisture that might cause rust under the mounts.