The advantage of the little sealed packets is that they stay sealed almost indefinitely and happily put up with living in a pocket, whereas DIY bags don't tend to either seal so well or are less robust.Could you get a big bag and get some small/tiny ziplock bags and decant your own? Or would the big bag be an unwanted temptation?
Unfortunately my children seem to be fond of skittles so they regularly raid my in-car supply on the way back from school, I may need to visit ASDA soon!