This method makes use of the fact that non stackable items emit a higher comparator signal than stackables.
Take 5 dirt blocks(any 64 stack item) and a sword(any unstackable).
Put these 6 items in a single dropper and take another dropper, make these 2 face each other.
Every time the dropper fires, it will throw out a random item. We can take a comparator output from the second, and if there is line strength more than one, [insert deadly roulette trap here].
If u want it to be a one in six chance, give a signal to second dropper to reset it. If u want it to fire once every six times, make the second dropper output to a locked hopper instead, and using a comparator, detect when 1st dropper is empty and unlock the hopper to let the items in(don't directly unlock hopper from 1st observer comparator output,use a pulse extender to let all six items back).