Advisors and investors are not the same thing. All investors should advise, not all advisors should invest. Generally, you shouldn't cold-approach either, it's just not effective.
This is a space you know (hopefully!) so reach out to contacts whom you respect and start small. And remember, you are the one asking for a favor with very little to offer in return right now.
Don't ask them for a day or an afternoon or to serve on a Board of Advisors and sign an NDA, etc.
Instead, find an event, meetup, etc you're both attending and ask them to meet up an hour early for coffee. It's time-bounded and not out of their way.
Regardless of how that goes, thank them via email or phone afterwards. Even if they don't know how to help you or it's not a good match, civility and politeness wins the day.
If it does work, invite them to coffee or dinner again in a month.
At every step, be clear that you're looking for advice. Better, if you take their advice, let them know that you did and how it worked out.
Throughout this, you should be prepared for a few things:
- You should always buy the coffee, lunch, etc. After all, you're getting the favor.
- Some people are/get overwhelmed with requests like this. Just because they decline now (or later) doesn't necessarily reflect on you or your idea.
- Some people believe they have nothing to offer. Sometimes a conversation or two can fix that.. if not, you'll want to find someone else. If they don't believe in themselves, they can be toxic.
- Everyone has advice. At the end of the day, it's your job to figure out which pieces do/don't fit and how to apply them.
- Everyone has advice. When you don't take someone's advice, the self-confident people won't worry about it.. the doubtful people may think that reflects on them. Be careful of bruised egos.