An event, virtual or otherwise, is organized around programming--packaged content. The audience then is contingent on the content. Start thinking about businesses that support and benefit when complementing either the content and/or the audience. Conduct an event without a sponsor, first. Capture metrics be them social, web, mobile or all of the above. Put together and media one sheet for potential sponsors and put some feelers out there. Indirectly pitch to sponsors you think could serve and support your growing community, and in turn, ask the community about their thinking around sponsorship: whether they are for or against, and if the former, who they think would be a good sponsor.

By way of an example, take a look at what MackCollier is doing with #blogchat on twitter. Blog chat started this year. In a few short months, Mack was approached by sponsors interested in supporting the conversation he was leading. In his own words on the cusp of his first sponsored blogchat:

"#Blogchat will have its first sponsor this weekend. This might be the first time an existing twitter chat has been sponsored, not sure if #journchat has beaten me to the punch there or not. Moving forward, there will be a maximum of one sponsored #Blogchat each month.
For me, bringing on sponsors isn’t about ‘cashing out’ so much as finding a way to use the sponsorships to improve the #Blogchat experience. From the participants’ point of view, they shouldn’t really notice any difference between a sponsored and non-sponsored #Blogchat. The topics will be completely consistent. From the sponsor’s point of view, they get a chance to interact with #Blogchat participants, and that increases their level of exposure and access to potential customers.
And for me, bringing in sponsors means I can spend more time on growing #Blogchat and bettering the chat for its community. Unfortunately, my own work with clients means that I can’t spend as much time as I’d like with the #Blogchat community throughout the week, and hopefully bringing in sponsors will help alleviate this problem for me.
But the key question I asked from the start was ‘If I bring in sponsors, how can I do that PLUS improve the #Blogchat experience for its participants?’. I think as long as I can do that, it’s a win-win for everyone. I am sure there will be some pushback and there always is whenever monetization enters into the equation with social media. But so far, the #Blogchat community has been supportive of this, and I think they’ll see Sunday that this is something that will ultimately benefit them."

Ultimately, start thinking of your self as a community facilitator. When you make your ask, or when you're approached, community support should be your top priority and everything else will take care of itself.

I hope this helps.