Hi Cindy,
What type of niche is your client in? Because that will have an impact on the best way to approach potential jv partners.

LinkedIn is great but there are other platforms. I was initially surprised by how much networking occurs on Facebook. See if the companies you're interested in working with have FB fan pages and check them out. And like anything else, perhaps just make a few posts, replies and comments on those pages before reaching out for a potential partnership.