If you dont have it, get liability insurance first thing. That is the most important tool. Then just make a couple of calls to a few companies that you respect. Not too many. My price is is set on how hungry I am and how does it compete with working my own jobs. They get the credit for your work. That is a big downside, you also have no contact with the customer whatsoever. Your customer is the contractor.
In a sense, this is usefull because it leads to networking amongst tree companies and opportunities for you to learn from them and them to learn from you. Everyone in this business has some cool tricks of their own. We shouldnt necessarily view eachother as competition. they obviously cant climb trees like you, so you are in a different businesses, they are in the debris management business. You are hiring them to do your dirty work. They also do the hard work of getting jobs, dealing with money, just so you can maintain your climbing habit. It is important to maintain your independence though.
The hard thing I find about subbing is staying engaged in that process. It can be hard to say "no, i wont cut that tree down" Or "its snowing and icy and I am not going outside today for once". It can be difficult to get people to put on hard hats or safety glasses or call "underneath!". You have to though. It is your insurance your working on. There are a few companies that I really like the people, but no one ever puts on a hardhat and it makes me too uncomfortable. My price is too high for them now.
Just bring your best. Use your own equipment, let it be known that if the ground guy puts a chainsaw through your rope, it needs to be replaced. Climb some trees.