Wow! I'm not sure what to suggest, but my first thoughts were to contact people about using the fairgrounds in Champaign or contacting the park districts about using Hessel Park, Crystal Lake Park, or one of the meeting centers in Champaign. Also, you might ask through the alliance if someone is willing to co-sponsor this. In my personal experience, some of the churches in Champaign-Urbana might be willing to help.
Other parks might be possibilities as well. It's been my experience to use Hessel Park for Safety Day and Concerts in the Park. The C-U Duck Race is held yearly at Crystal Lake Park, and they do concerts weekly in good weather. The Lake House is open for rental. Also, you might want to check into West Side park in the middle of Champaign. They hold the Taste of Champaign and the National Night Out celebrations there every summer. These are my best suggestions. Let me know if we can be of any further assistance.