I've met co-founders and team members through participating in Startup Weekend evens. (The last two companies I sold were started at startup weekends!) But, I generally recommend participating in others groups or pitching different ideas than you're trying to find a co-founder for.
Another great resource is FounderDating.com Just remember, don't think of it as speed dating... Think of it as Match.com and trying to find a lifelong partner.
Under certain circumstances, I and some of my colleagues would be looking for joint ventures. But what we'd bring to the table specifically would be online property – i.e. domain names. That would require a very compelling project, though, and a coherent business plan.
I realize that's not quite what you're asking for. But there's a 10% chance it may be relevant to your business idea – or to someone else in a similar position.
Personally, I'm also on the lookout for interesting projects where I could act as a minor partner, contributing branding help, copy, and perhaps legwork. I'll be rolling out a few collaborative projects like that with other primary entrepreneurs in the next couple of years.
Again, that's probably not an exact fit for you. But you asked about places to meet collaborators, and one such place is right here.
Over the years as a lawyer and a business consultant, and recently in my new role as the VP business Development of a software dev shop I have met a lot of entrepreneurs with the same questions.
The answer of where to find your co-founder depends on who you are, what you are looking for and what is your business idea.
In order to find a good co-founder FOR YOU - you need to start with good chemistry between the two of you.
Seconly, you will need someone who can complete you. Ask yourself what will be your job in the new venture?
What strengths do you have and what qualities experience and capabilities you will want your co-founder to have.
Start with you friends and see if one (or more) of them can be your co-founder. It will - in most cases - be your safest bet.
You can search for the skills, profiles you are looking for and connect with them. You can communicate and see if they are as excited about your business idea as you are. Most people like to take things forward on the phone and in-person meeting later. I consider it the best and most efficient channel today.
My local meetup groups have been key to my success in connecting with others who have similar interests. I also say "yes" to networking groups and fundraisers that are outside what I would normally attend. I do this to create diversity within my network so I benefit from new perspectives. For example, a "Fancy Hat Ladies Tea" fundraiser that supports a dog shelter may be outside the tech industry, but I benefit in forming new connections, plus I gain valuable info on how to help my two clients who serve dog owners. One client operates a local dog boarding/grooming business and the other is a boutique store with locations on the Las Vegas Strip and the Town Square mall.