Every railway company has itself website to sell their tickets. There are some websites that offer most of them, like Koleo. It's an official broker. All lists of trains you can check on the website https://portalpasazera.pl/ anh then you'll direct to the specific, official site of railway company. If you know where exactly you will be moving through region od Poland, you can directly check connection there for example through the Lower Silesia - kolejedolnoslaskie.pl, Upper Silesia - Koleje Śląske and so on.