Now, whenever you could not find the exact voltage of the capacitor from electronic shops, you can always use a higher than the original rated voltage of capacitor for replacement. The best suggestion that i can give you is to use a next voltage level. For example, the capacitor rated voltage ranges that can be found in the market are 6.3v, 10v, 16 v, 25 v, 35v, 50v, 63v, 100, 160, 200v, 250v, 350v, 400v, 450v and etc. If you want to find replacement for the 25volt range cap then use the 35 volt. Of course you can use 50 or even 100volt but do you know that the higher the voltage of the cap, the higher the price? The capacitor price is not base on the capacitance value!
Well, you can use the 160v or even 250 volt range but it is not practical. The price is expensive and there will be no room/space for you to insert the cap. You may have to modify and use longer wires to fit in the big cap and this will look ugly. Generally the higher the cap voltage, the bigger is the capacitor size. Sometimes it depends on the manufacturers too. I came across capacitor that was rated 50 volt that have smaller size than a 25 volt cap!
Actually finding cap for replacement is not an issue, if the replacement cap can fit well into the original place then your job is considered done. This post is not for only electrolytic capacitors, it can be used for the non polar caps as well.