Liability insurance is often an additional cost on top of your regular farm insurance. The best place to start when looking for coverage is starting with your farm agent, but be aware that not all companies want to jump into the agritourism market. One place to look is with your Farm Bureau to see if they have an agency to recommend. Another might be a broker familiar with agritourism.
Be careful you don't start shopping around too much as insurance companies often confer with each other and you can easily get tagged as high risk (not because you are but because the activities in agritourism are untested in courts so the insurance companies are wary of coverage).
When looking at your farm in terms of safety, we have found this awesome resource put out by the National Children's Center for Rural and Agricultural Health and Safety at http://www.safeagritourism.com/. It provides written and visual examples with Walkthoughs, Checklists, and Resources. Insurance is necessary but limiting your liability by being aware of safety issues in areas where visitors are present is crucial as well. You might also check out this chapter on Risk Management from the UVM Agritourism Guide
Photo by Dennis Rivera