Python: validating the existence of NLTK data with database search
NLTK has a built-in NER model that would extract potential Organizations from text, you can read about it here (and see examples) NLTK book (look for section "5 Named Entity Recognition").

However, if your input text has organizations in a very specific context that wasn't seen by NLTK NER model, performance might be quite low. In that case you should be looking into training your own NER model, what would extract company names. For that you would require to manually markup a small amount of your dataset.