What you need is Public Liability Insurance. This insures you in the event that a member of the public coming on your work premises injures themselves. The cost will be very dependant on the nature of what you (and the public) do. Try speaking to a business insurance agent - loads in Yellow Pages - they will know what you require.
Indemnity insurance insures you against consequential damages by a client / customer in the event you do something wrong which impacts them - this is not what you require.