Move the rear sight in the direction you want the strike of the bullet to move. If you are shooting too far left, move the rear sight to the right. There is a formula on how much to move it but in practical terms, a tap and another tap and a half is easiest. Regardless, use a pencil to make a witness mark from the sight to the frame, that way, you can always come back to where you started. Formula is:
(sight radius)(error)/distance to target
All units must be the same.
To your other question, there are many adjustable sights out there, Pachmyer come to mind and will fit in the dove-tail where your rear sight fits. If it is oversize, take a stone of fine file and work at the bottom gradually until it taps in firmly. No need to take it to a smith. If you go too far and it tends to move, stipple the bottom with a punch to give the friction needed to keep it in place.
