Very interesting question. I'll try to explain. Such grammatical structures, where verb subject is hidden (omitted), and intonation of sentence is high, are used more often in cases, when you intend to stress your negative emotions or displeasure to a person's actions. Your sentences should be written correctly so: "Нашла, с кем разговаривать! Радио лучше слушай! И-то толку больше будет!" Here "она" in "Нашла (она)" is omitted. This sentence has negative context: "Она" (she) should do better something else (listen to the radio), and hasn't to talk to somebody (she or he - the sentence doesn't contain gender suggestion, only - "с кем" - who). In phrase "Нашла, с кем..." you should understand "с кем" as somebody, who is very undesirable person. For example, if I build a sentence like "Придумал (ты - omitted), о чем поговорить!" or "Ну ты и придумал, о чем поговорить!" - "(You) have devised what we need to talk about!"="You have bad idea about something we need to talk" , I want to stress my negative emotions about speaking theme. Neagative emotions are not only disagreeable theme or undesirable person, but also very difficult theme I can't understand etc. Conclusion: such grammatical structures are mostly used to express our negative emotions about somebody want to do any actions or something that will happen. Mostly used verbs: придумать (придумал/придумала/придумали), найти (нашел/нашла/нашли), выдумать (выдумал/выдумала/выдумали), захотеть (захотел/захотела/захотели) etc. To emphasize your emotions (astonishment sometimes) you can use particles (interjections) "ишь", "ну и": "Ишь придумал!", "Ну и придумал!". "Ишь" can be used not only for something negative, but also for wonder/amazement witt (little) envy. I hope, this will help a bit :)