On the graveyard path mission in Lychfield Graveyard there are two silver keys hidden in the graveyard, one in the water, the other hidden in a grave. Once you have found these keys do a Hero Save and load up the autosave before the mission. (You will still have the keys you found earlier on) Go and look for the keys you found earlier on and they will still be there. Keep on repeatins this and soon you will have 20 silver keys.