Gear Aid deserves a whole chapter on their various outdoor gear repair kits. When your buckles give way, this simple slotted replacement buckle should fix you up.
It takes a little finesse to twist it onto your existing nylon loop, but it seems to be as strong as a solid buckle, and not easy to undo either. The military uses them, and they know how to break things.
Their basic buckle kit includes a 1″ Side Buckle set, 1″ Tension-lock ladder-type buckle and 2 Slik Clips.