Please help!! want to cry..how do deal with difficult people... people with semi-authority? (university)?

my department hires someone to do our calculations. this person has a position in that he's the head of his own department, but is unaffiliated with my dept. i'm still a student but im forced to ask him for help (i think its cuz my boss hates him too and would rather let me deal with the guy). our dept pay's his. since i am young and unfamilar with the other department, this person treats me like dirt. he's condescending and rude. he expects me to know his dept, when i am only doing my own job (im an expert at my own dept). recently, i asked him for something. it's been a week, he has not responded. he is delaying my work! i emailed him to ask him if i should make an appointment with him again just so he can tell me a number that he already calculated (it would take him 5 mins!). nevertheless, he ignores my email cuz he's a jerk. what should i do?!?!?! i need the number! i have been nice all the way through. he is the one who personally attacks me, yet he still continue to be rude!!!