I believe Ask HN is a pretty valid place for that if you're looking for feedback in the idea stage. Other than that, gather a few of you most important questions needed to validate your idea and have a brief product summary and hit the streets asking for people's pinions. I would do a few different group types. Start with whomever you believe would be your target audience, go to places where you would find that audience type, and start mingling. The other set should be a populated unbiased public place to get as random of a selection as possible. The point of this is that with your first group, your thoughts may be confirmed that they are your audience and that your idea is solid. However, when surveying the second group, you may find that there's a lot of interest in general or in a different demographic. You may find that another group of people even like it more or you may find that nobody likes out and that your initial group is your primary audience. On the flip side, your first group may not like it at all, but you may find a lot of people that like the idea in the general group. In that case you know your initial predictions were wrong.

After going after those two groups, I would then go for a second round. This time it could be a number of things. If your target audience turned out to be different than you thought or if you have more than one target audience than you nay want to go to places where you'd find a dense populous of those people and ask me people the same Quaid to confirm the result. The other thing you may want to consider is to come up with a different set of questions to help refine your idea. Would you like product A our B better? What features would you like to see? Would you add, change, or remove anything? Just try to get a better sense of want people need and would like to see.

Lastly, when doing this it would also be a great to start collecting emails from people interested for when you launch our if you want to get further feedback from them.

This is probably the most important stage if in any product life because it will determine if you're idea is needed or wanted, how much of a demand there is, and get your product on the right track so you're not developing features people don't want and wasting time/money. Don't be lazy with this stage. Do your due diligence.

Why not posting your idea here? It might pique some interest here.