You might want to try Ad Swaps with some websites that coincide with your related niche. I remember I saw some nice bidding websites and checked them out from some forum banner advertisements. Hope this helps!
Keep the End in Mind, You already have an intention on getting more paid members to your site. Where does your target market go to hangout? Put your message in front of them. Setup a blog for your business and drive traffic to your blog, What to put on the blog? What kind of problems does your target market have? Address a solution to the problem that ends up being your business. Make sure you are building a list and send valuable information to your list daily. If you help enough people to get what they want, You will in turn get what you want.