Look for agents that specialize in commercial liability. When I worked for myself, I had an excellent agent. Not sure what company he wrote me with, didn't matter. Ask other tree guys locally who they use. Ask your stump guy, etc.
Unless you are working on Government property or something, $1 mill. is overkill. I'd go with an amount over the total value of the average home you work at. I had $300,000 and never had a problem. Most houses I worked at were worth substancially less than that, although some were a little more. But I explained to the homeowner that my chances of totally wiping out their entire home and property were fairly slim without a lot of dynamite!
In 10+ years I had one claim, for an air conditioning compressor. Was about $1200, and I paid the $500 deductible. Ins coughed up $700. No problems, they didn't even raise my rates the next year.
Oh, if you want to check it out, there is a thread in 'Off Topic' forum about insurance.