You could take the sqaure root of random number then round it, this will make lower numbers rarer, but you could minus that value from 100 so that higher values are rarer.
100 - (round(random(10000)^0.5))
or you could make it even harder to get 100
100 - (round(random(1000000)^0.33))
That works for me, don't know if thats the best way to do it though.Here's a cap showing the numbers.