Greetings Scoot!
It's funny you ask this question because I just spent most of the day going through my insurance policies. In regards to your question, you need to decide what kind of excavation you want to do. I ask this question because there is a big difference in how you will be classified by an insurer depending on the type of work you are doing/ intending to do. If your work will mainly consist of light excavation and no trenching (including digging deeper than 3' in the ground) then you might be able to get away with getting a landscaper's general liability policy with a light earthmoving endorsement. This option is usually thousands of dollars cheaper than getting a general liability policy for an "excavating" company.
If you decide to go the route of an excavation contractor, then you will need to get a commerical general liability policy with an excavation endorsement. You should talk with an insurance broker about your specific needs and what kind of policy you are eligible for. Usually an excavation policy will run in the several thousand dollar range and will increase exponentially as you add employees.
As a final note, make sure you shop/ask around for good excavation insurance brokers. Not all brokers are created equally, and to give you an example I went to three brokers and got a range of quotations from $2000 to $10000. I ended up going with a company called Selective Insurance and got a policy with excellent coverage and low deductibles at a very reasonable price.
Good luck!