Generic center filters are an option, but those basically try to copy the most common possibilities, and are designed for similar amounts of falloff, which you should know in advance with respect to your particular lens before selecting one. This topic has been discussed over and over again. Look up some or Bob Salomon's posts on the subject; he's fairly expert at it, and would instantly contradict the notion that you can just go out and buy anything that fits the filter thread and walk away with a correct result.