I have bought 3 boxes of these shells for my 12 gauge. They do help. They make ones that are black for sunny days and orange for cloudy days. I have used them for trap and they have helped. The way I was told to use them is only use a few in a round of trap (mix them in with the other shells or just use a few to see what your doing wrong). You don't really want to blow a whole box of them at once especially since they are 15 bucks a box. I found them only to really work for straightway. They are useless on oscillating trap or skeet.
I have seen them at Canadian Tire but I bought mine from the range I go to.
I would recommend these for new shooters or even when you buy a new shotgun.
