Where can I find the sources on which the opinion that says gelatin from non kasher animals is permitted based on the concept of nifsal me'achilas kelev?
These three all say some variation of that theme:
Chaim Ozer Gradinski. Shut Achiezer III:33
Tzvi Pesach Frank. Har Tzvi, Yoreh Deah 83
Ovadia Yosef. Yabia Omer, Yoreh Deah, 8:11