Senior Engineers, friends who have experience, past professors. But really the first person that you should go to and the only one you'll really need is you.
Learn how to learn, there are plenty of resources out there to help you:
1) Research papers, google scholar and ieee amongst others go a long way. You can find out what has been done and it will help extrapolate knowledge if you want to do something new.
2) Books and tutorials. The art of electronics will get you pretty far, there are other great books that are worth mentioning like Henry Otts Electromagnetic Compatibility engineering. Find out what books you need to increase your knowledge for your job. If you have to go to a university or (good) local library and start looking through EE books, especially those that will help you at your job.
3) Learn how to google. I've been amazed by how much I don't know how to google, and I search 50+ times a day. I've thought I did a through search on a subject, and then I change a few words and find even better info (like I found a supplier to help advance our product months after I had done a multiday search.)
4) Get working knowledge, you learn by doing. Find some projects to do at home that parallel your work and will help you fill in knowledge, its always good to prototype a PCB, learn new software or pick up a dev board and play around with it. If you need motivation, tell yourself you'll sell it on kickstarter and then don't. There are plenty of tutorials on the internet to help you get started.
University doesn't really give you a lot of working knowledge unless you took a lot of lab classes or had a research internship or job while going to school.
If you can be successful at learning how to learn you'll get promotions and people will think you are smart. It takes time and effort on your part, and some guts.
You'll notice that there are people on this site that don't know how to tie their own shoes, they don't care about learning how to learn, they want the answer given to them, but they are depriving themselves of valuable learning. Also, learn how to communicate. Learn how to explain concepts you've learn, apply those concepts to real world problems.
Also learn how to design, there is an art to it, a lot of people in industry gloss over it. You should be able to predict what is going to happen, produce it and have it work like you expect most of the time. There are a lot of people that don't care about this process, they will make changes until they think it works and don't take the time to actually find out the underlying problem.