They are 6-48 for the rear receiver screws and 6-40 for the front sight screw. Go to a gun shop/smith and see if you can get the correct screws. Almost impossible to buy other wise. I'm in the same boat as you are..
Most kits that I've seen are gunsmith sets and run in the 25-50.00 range. I THINK that's what you'll find. I sure couldn't find cheaper. Good luck, as for me I'd rather spend a few bucks for a few screws than that much..
Be safe, CraigJS