not sure if anyone mentioned this but there is a sure fire way to get 4 runes in a matter of 1-2 minutes
start a new character and go into bronze, go to their initial chest and you will be sure to find 2 runes, save and exit then do the same thing on bronze, save and exit again then go into a free world game and dump them into your hero chest (or summon your imp when youre in the silver game if you have the 360 version)
then delete the character and do it again ... you can get hundreds of runes in no time by doing this