If you're at ground-level and just starting out, you can buy your way into a few (as jmduke pointed one out). I highly recommend joining Rob Walling's (rwalling) Micropreneur Academy and branching out from there. The cover charge is a bit pricey at ~$50 a month, but if you contribute to discussions and look for ways to help other people (this is key), you'll find your way forward into some great communities.
At $50 per month, it certainly seems like a good way to keep the owners employed, at least.