I bought my CMMG AR15-22 from a local gun shop. He sold it to me for $450. It is a Standard CMMG lower with a dedicated .22 upper. I went this route incase I wanted to swap uppers. I also have owned the M&P15-22, it was a fun accurate rifle too.
Here is the link to the gunshop that had the CMMG. He used to sell a lot on gunbroker. I am not sure if he has anymore in stock, as I bought mine a year and a half ago, but he did have one or two pre panic. It is a little LGS, that has always been great to me..