I was the president of a club at my University for 2 years and we used to host multiple autocrosses per term. As MTCRX said, typically sites require a minimum 1,000,000 umbrella insurance policy.
Usually a good place to start is google maps to try to find a lot the appropriate size, then scout it out to make sure the pavement is suitable. When you find a place you'd like to shoot for contact their facilities manager to see if they are willing to rent out the lot. Depending on lot condition and what the place of business is, you may be able to get them to help out with cleaning the place off for you. We used to use GM facilities and they would come by with their street sweepers that day before and clean off as much loose gravel and debris as possible.
To set up an appropriate course design using the space you have available, consult the SCCA course design manual. This will be your best chance to set up an appropriate and safe autocross course.
Also based on the policy, some will allow multiple cars on course at a time, to keep the cost down you can get a policy which covers 1-car at a time, so that may dictate event size and course design.
We went through K&K Motorsports Insurance group. I would start with the insurance contact to understand what is involved, I know prior to events we would have to turn in a club membership list and our insurance rates were based upon the number of members, they will then provide the legal liability waivers for the event.
http://www.kandkinsurance.com/Motors...-Sponsors.aspx
Most autocrossers follow SCCA classing guidelines, but I know our local Detroit Sports Car Council runs their own classes, similar to SCCA but some key differences.