My cats are removing their collars!! Please help!!?

Now to start with, they are around 4 months old, both boys. I have never used the detachable collars or "breakaway" collars because all the cats I've ever had just bite the clasp and it comes sliding off, we used to use nice nylon collars with a proper buckles, none of my cats could ever get those off. now with the addition of these new kittens, i went looking for collars.. and somehow.. in my absence, the cat collar section has reduced down dramatically. And all of them are breakway. does anyone now where i can get some nice proper cat collars? i let my cats outside during the day and i want the kittens to have a collar that wont fall off when i start letting them out.