Each Key Broker. You can get advanced directly by getting your own Key Broker contract. To get that you need to remit proof of 150 cases written through all companies over the past 12 months. Then you have to submit 100 approved cases over the next 12 months. After that you write your first 12 deals as-earned, then you get a 9 month advance.
If you're with a Key Broker they can choose offer an advance at their will. If you write more than 2 cases during the last 6 months they have to release you. Most won't and GR has the most horrible agent situation I've heard of. None of this is discussed upfront. If you go with GR direct just as an agent then choose to go under a Key Broker to get an advance, they will not release you.
If you go and get an advance through a Key Broker you'd better damned well know and trust him. He gets your commission and cuts you a check. They can change your advance at will so if you start to get a few chargebacks and they get nervous they can simply hang onto your money.
You need to get appointed directly and quit having people take a slice of your production.
I would avoid advances - think of credit card debt... same thing, it is not your money.