What’s the best way to get in contact with Network Operators and Engineers?

Hi, I’m a UX designer in a startup and I'm doing a user research study about SDN products (It's basically interviews about the needs and pain-points of Network Operators and Engineers). I already created the application form ( http://svy.mk/1MgC8gt ) and every interviewee will get a $100 VISA gift card, but now the questions is: Do you have an idea how I can get participants for these interviews? I already tried searching and inviting potential candidates via Linkedin (takes way too long) and Twitter (almost impossible to find the right candidates). I appreciate all the help I can get. Thanks very much in advance!