Given the harsh burden of being the first line of defense against the Sun's heat and rain/snow/wind, a rainfly will wear out faster than other components.  It is cost-effective to manufacture the rainfly at the same time we produce the tent.  In our quest to constantly improve our tents, dimensions and attachment methods change from one model to another, and sometimes from one year to another.  A rainfly from a similar looking tent or newer tent probably will not fit a tent that is a couple of years older. Unfortunately, there are not many options available if your rainfly is missing, torn, or UV damaged.