Go talk to your Farm Bureau, the farm insurance we have covers liabilities of anyone or even livestock, that comes onto the place that gets hurt.  There are several add ons you can put on as necessary, when we were training I had the liability insurance with a write on for boarding.  As far as the horses being insured, you would have to put that in your contract requiring the owners to have it on the horses before you took them.  It gets to be a touchy thing when you get into boarding and training horses.  People today have just become to suit happy.

Hope that this helps,

Easty