I don't know what exactly you are trying to research, and for most purposes you will obviously need to get access to some literature about the subjects, so a library would be your best starting point, but for a preliminary overview an online search is not a bad idea. However, a WW2 site is not really the best starting point for something that happened a good 20 years before WW2, and the English wikipedia is probably not the best site for detailed information about early 20th century Germany. I only know the very basics about the Freikorps, but for an online search maybe using google translate for the relevant German wikipedia pages might be a better start (assuming you don't speak German).

Wilhelm Reinhard https://de-m-wikipedia-org.translate.goog/wiki/Wilhelm_Reinhard_(General)?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

Reinhard’s freikorp https://de-m-wikipedia-org.translate.goog/wiki/Freiwilligen-Regiment_Reinhard?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

Kiel iron brigade https://de-m-wikipedia-org.translate.goog/wiki/1._Marine-Brigade?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

Potsdam regiment https://de-m-wikipedia-org.translate.goog/wiki/Regiment_Potsdam?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

Franz von Stephani https://de-m-wikipedia-org.translate.goog/wiki/Franz_von_Stephani?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp