A big problem in the REC markets is lack of price transparency. I worked for a startup a while back building a trading platform to try to get major players together at a regular interval to agree upon pricing that could then be executed upon and published publicly. The idea didn't pan out; despite having a lot of the biggest traders in the game on board, there just wasn't enough trading to make a company viable. I believe that this is due in part by two things. 1 - the large variance in regulations. You can't just trade a REC for a REC. If there was national consensus then it would open up much more liquidity. 2 - most RECs are pre bought/sold as a means of project finance. A company building a wind farm will pre sell years in advance to help fund the hardware. On the other side utilities will pre buy all the credits to cover their needs. There is relatively low day trading, making a technology solution difficult.

Currently most large scale traders pick up the phone and call brokers like the good old days of the stock market. So to answer your question, yes you need to request a quote from each. There are also bloomberg streams for pricing and some brokering companies publish indexes that they believe reflect the current pricing.