Any sight that fits your size dovetail (from Marble's etc.) will work (like this one) -
http://www.midwayusa.com/viewproduct...tnumber=365177
As far as which height to get, there is a calculator/chart on the Brownell's site (may be one on Midway's too).
For the screw, I would just get a piece of threaded screw the appropriate size, cut it to the right length and put a knob on it (your existing one if you can get it off, or make one).
Don't know of any source for Redfield sight spares (other than junk boxes at gun shows etc.).
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