So we all know bull horns is needed for us to be able to do mission is Spain. So the question right now is where we can get more bull horns in regular missions. The answer is Vanquish Oelebeth in Nightshade mission. Based on my experience it gives 8-7 bull horns in 1 of 3 attempts. I'm not sure if this is always the case. If you have any knowledge regarding this please post it in the comment area.