Where should you look for feedback? The short answer is that you should seek advice from anyone you think should be interested in your company, whether you see them as potential or actual customers, investors, or people who have done things you respect that are related to your own or your company's goals.

I have asked speakers at conferences, school teachers, game developers, VCs, and people I've met at pitch contests to give me feedback on specific problems or aspects of my company. Usually I ask them one question at a time, and I always respond to their feedback. This is how I found all my official advisors; generally if I find myself pestering them constantly I will ask them if they want to become more involved.

Customers should be easy to identify. Finding good advisors is harder, especially if you're from a small town. Go to startup events and reach out to people online. A lot of successful entrepreneurs and VCs have blogs and will answer questions via email.

You will sometimes outgrow your advisors, so you should always be on the lookout for people who can help you. No one person will be able to provide insight on everything. In general, you should look for people who are really good at strategy, marketing, and negotiation, as well as anything you're weak at (public speaking or recruiting, for example).

You will end up with a lot of feedback if you try. You need a way of evaluating and prioritizing it all, because it is sometimes going to contradict your own beliefs and other advice you get. Also, some of the best advice won't necessarily be something you can act on immediately, but you will want a way to remember it and use it to help shape future plans.

My rules of thumb for feedback:

if it makes sense, do it.
if it doesn't make sense, ask someone else's opinion.
if you can't execute on it, that doesn't make it bad advice.
plan to execute on good advice in the future.
never be confrontational. You are the decision maker, there's no reason to argue with people if you don't agree with them, and you'll look like a jerk. Assume that people who give you advice are just trying to help.
always say thank you and follow up with people if you follow their suggestions and they work out - everyone loves to hear about success.